A Triangulation Checklist Born From the Question; “Why NOT Use Close Relatives for Triangulation?”

One of my readers asked why we don’t use close relatives for triangulation.

This is a great question because not using close relatives for triangulation seems counter-intuitive.

I used to ask my kids and eventually my students and customers if they wanted the quick short answer or the longer educational answer.

The short answer is “because close relatives are too close to reliably form the third leg of the triangle.” Since you share so much DNA with close relatives, someone matching you who is identical by chance can also match them for exactly the same reason.

If you trust me and you’re good with that answer, wonderful. But I hope you’ll keep reading because there’s so much to consider, not to mention a few gotchas. I’ll share my methodology, techniques, and workarounds.

We’ll also discuss absolutely wonderful ways to utilize close relatives in the genetic genealogical process – just not for triangulation.

At the end of this article, I’ve provided a working triangulation checklist for you to use when evaluating your matches.

Let’s go!

The Step-by-Step Educational Answer😊

Some people see “evidence” they believe conflicts with the concept that you should not use close relatives for triangulation. I understand that, because I’ve gone down that rathole too, so I’m providing the “educational answer” that explains exactly WHY you should not use close relatives for triangulation – and what you should do.

Of course, we need to answer the question, “Who actually are close relatives?”

I’ll explain the best ways to best utilize close relatives in genetic genealogy, and why some matches are deceptive.

You’ll need to understand the underpinnings of DNA inheritance and also of how the different vendors handle DNA matching behind the scenes.

The purpose of autosomal DNA triangulation is to confirm that a segment is passed down from a particular ancestor to you and a specific set of your matches.

Triangulation, of course, implies 3, so at least three people must all match each other on a reasonably sized portion of the same DNA segment for triangulation to occur.

Matching just one person only provides you with one path to that common ancestor. It’s possible that you match that person due to a different ancestor that you aren’t aware of, or due to chance recombination of DNA.

It’s possible that your or your match inherited part of that DNA from your maternal side and part from your paternal side, meaning that you are matching that other person’s DNA by chance.

I wrote about identical by descent (IBD), which is an accurate genealogically meaningful match, and identical by chance (IBC) which is a false match, in the article Concepts – Identical by…Descent, State, Population and Chance.

I really want you to understand why close relatives really shouldn’t be used for triangulation, and HOW close relative matches should be used, so we’re going to discuss all of the factors that affect and influence this topic – both the obvious and little-understood.

  • Legitimate Matches
  • Inheritance and Triangulation
  • Parental Cross-Matching
  • Parental Phasing
  • Automatic Phasing at FamilyTreeDNA
  • Parental Phasing Caveats
  • Pedigree Collapse
  • Endogamy
  • How Many Identical-by-Chance Matches Will I Have?
  • DNA Doesn’t Skip Generations (Seriously, It Doesn’t)
  • Your Parents Have DNA That You Don’t (And How to Use It)
  • No DNA Match Doesn’t Mean You’re Not Related
  • Imputation
  • Ancestry Issues and Workarounds
  • Testing Close Relatives is VERY Useful – Just Not for Triangulation
  • Triangulated Matches
  • Building Triangulation Evidence – Ingredients and a Recipe
  • Aunts/Uncles
  • Siblings
  • How False Positives Work and How to Avoid Them
  • Distant Cousins Are Best for Triangulation & Here’s Why
  • Where Are We? A Triangulation Checklist for You!
  • The Bottom Line

Don’t worry, these sections are logical and concise. I considered making this into multiple articles, but I really want it in one place for you. I’ve created lots of graphics with examples to help out.

Let’s start by dispelling a myth.

DNA Doesn’t Skip Generations!

Recently, someone emailed to let me know that they had “stopped listening to me” in a presentation when I said that if a match did not also match one of your parents, it was a false match. That person informed me that they had worked on their tree for three years at Ancestry and they have “proof” of DNA skipping generations.

Nope, sorry. That really doesn’t happen, but there are circumstances when a person who doesn’t understand either how DNA works, or how the vendor they are using presents DNA results could misunderstand or misinterpret the results.

You can watch my presentation, RootsTech session, DNA Triangulation: What, Why and How, for free here. I’m thrilled that this session is now being used in courses at two different universities.

DNA really doesn’t skip generations. You CANNOT inherit DNA that your parents didn’t have.

Full stop.

Your children cannot inherit DNA from you that you don’t carry. If you don’t have that DNA, your children and their descendants can’t have it either, at least not from you. They of course do inherit DNA from their other parent.

I think historically, the “skipping generations” commentary was connected to traits. For example, Susie has dimples (or whatever) and so did her maternal grandmother, but her mother did not, so Susie’s dimples were said to have “skipped a generation.” Of course, we don’t know anything about Susie’s other grandparents, if Susie’s parents share ancestors, recessive/dominant genes or even how many genetic locations are involved with the inheritance of “dimples,” but I digress.

DNA skipping generations is a fallacy.

You cannot legitimately match someone that your parent does not, at least not through that parent’s side of the tree.

But here’s the caveat. You can’t match someone one of your parents doesn’t with the rare exception of:

  • Relatively recent pedigree collapse that occurs when you have the same ancestors on both sides of your tree, meaning your parents are related, AND
  • The process of recombination just happened to split and recombine a segment of DNA in segments too small for your match to match your parents individually, but large enough when recombined to match you.

We’ll talk about that more in a minute.

However, the person working with Ancestry trees can’t make this determination because Ancestry doesn’t provide segment information. Ancestry also handles DNA differently than other vendors, which we’ll also discuss shortly.

We’ll review all of this, but let’s start at the beginning and explain how to determine if our matches are legitimate, or not.

Legitimate Matches

Legitimate matches occur when the DNA of your ancestor is passed from that ancestor to their descendants, and eventually to you and a match in an unbroken pathway.

Unbroken means that every ancestor between you and that ancestor carried and then passed on the segment of the ancestor’s DNA that you carry today. The same is true for your match who carries the same segment of DNA from your common ancestor.

False positive matches occur when the DNA of a male and female combine randomly to look like a legitimate match to someone else.

Thankfully, there are ways to tell the difference.

Inheritance and Triangulation

Remember, you inherit two copies of each of your chromosomes 1-22, one copy from your mother and one from your father. You inherit half of the DNA that each parent carries, but it’s mixed together in you so the labs can’t readily tell which nucleotide, A, C, T, or G you received from which parent. I’m showing your maternal and paternal DNA in the graphic below, stacked neatly together in a column – but in reality, it could be AC in one position and CA in the next.

For matching all that matters is the nucleotide that matches your match is present in one of those two locations. In this case, A for your mother’s side and C for your father’s side. If you’re interested, you can read more about that in the article, Hit a Genealogy Home Run Using Your Double-Sided Two-Faced Chromosomes While Avoiding Imposters.

You can see in this example that you inherited all As from your Mom and all Cs from your Dad.

  • A legitimate maternal match would match you on all As on this particular example segment.
  • A legitimate paternal match would match you on all Cs on this particular segment.
  • A false positive match will match you on some random combination of As and Cs that make it look like they match you legitimately, but they don’t.
  • A false positive match will NOT match either your mother or your father.

To be very clear, technically a false positive match DOES match your DNA – but they don’t match your DNA because you share a common ancestor with your match. They match you because random recombination on their side causes you to match each other by chance.

In other words, if part of your DNA came from your Mom’s side and part from your Dad’s but it randomly fell in the correct positional order, you’d still match someone whose DNA was from only their mother or father’s side. That’s exactly the situation shown above and below.

Looking at our example again, it’s evident that your identical by chance (IBC) match’s A locations (1, 3, 5, 7 & 9) will match your Mom. C locations (2, 4, 6 8, & 10) will match your Dad, but the nonmatching segments interleaved in-between that match alternating parents will prevent your match from matching either of your parents. In other words, out of 10 contiguous locations in our example, your IBC match has 5 As alternated with 5 Cs, so they won’t match either of your parents who have 10 As or 10 Cs in a row.

This recombination effect can work in either direction. Either or both matching people’s DNA could be randomly mixed causing them to match each other, but not their parents.

Regardless of whose DNA is zigzagging back and forth between maternal and paternal, the match is not genealogical and does not confirm a common ancestor.

This is exactly why triangulation works and is crucial.

If you legitimately match a third person, shown below, on your maternal side, they will match you, your first legitimate maternal match, and your Mom because they carry all As. But they WON’T match the person who is matching you because they are identical by chance, shown in grey below.

The only person your identical by chance match matches in this group is you because they match you because of the chance recombination of parental DNA.

That third person WILL also match all other legitimate maternal matches on this segment.

In the graphic above, we see that while the grey identical by chance person matches you because of the random combination of As from your mother and Cs from your father, your legitimate maternal matches won’t match your identical by chance match.

This is the first step in identifying false matches.

Parental Cross-Matching

Removing the identical by chance match, and adding in the parents of your legitimate maternal match, we see that your maternal match, above, matches you because you both have all As inherited from one parent, not from a combination of both parents.

We know that because we can see the DNA of both parents of both matches in this example.

The ideal situation occurs when two people match and they have both had their parents tested. We need to see if each person matches the other person’s parents.

We can see that you do NOT match your match’s father and your match does NOT match your father.

You do match your match’s mother and your match does match your mother. I refer to this as Parental Cross-matching.

Your legitimate maternal matches will also match each other and your mother if she is available for testing.

All the people in yellow match each other, while the two parents in gray do not match any of your matches. An entire group of legitimate maternal matches on this segment, no matter how many, will all match each other.

If another person matches you and the other yellow people, you’ll still need to see if you match their parents, because if not, that means they are matching you on all As because their two parents DNA combined just happened, by chance, to contribute an A in all of those positions.

In this last example, your new match, in green, matches you, your legitimate match and both of your mothers, BUT, none of the four yellow people match either of the new match’s parents. You can see that the new green match inherited their As from the DNA of their mother and father both, randomly zigzagging back and forth.

The four yellow matches phase parentally as we just proved with cross matching to parents. The new match at first glance appears to be a legitimate match because they match all of the yellow people – but they aren’t because the yellow people don’t match the green person’s parents.

To tell the difference between legitimate matches and identical by chance matches, you need two things, in order.

  • Parental matching known as parental phasing along with parental cross-matching, if possible, AND
  • Legitimate identical by descent (IBD) triangulated matches

If you have the ability to perform parental matching, called phasing, that’s the easiest first step in eliminating identical by chance matches. However, few match pairs will have parents for everyone. You can use triangulation without parental phasing if parents aren’t available.

Let’s talk about both, including when and how close relatives can and cannot be used.

Parental Phasing

The technique of confirming your match to be legitimate by your match also matching one of your parents is called parental phasing.

If we have the parents of both people in a match pair available for matching, we can easily tell if the match does NOT match either parent. That’s Parental Cross Matching. If either match does NOT match one of the other person’s parents, the match is identical by chance, also known as a false positive.

See how easy that was!

If you, for example, is the only person in your match pair to have parents available, then you can parentally phase the match on your side if your match matches your parents. However, because your match’s parents are unavailable, your match to them cannon tbe verified as legitimate on their side. So you are not phased to their parents.

If you only have one of your parents available for matching, and your match does not match that parent, you CANNOT presume that because your match does NOT match that parent, the match is a legitimate match for the other, missing, parent.

There are four possible match conditions:

  • Maternal match
  • Paternal match
  • Matches neither parent which means the match is identical by chance meaning a false positive
  • Matches both parents in the case of pedigree collapse or endogamy

If two matching people do match one parent of both matches (parental cross-matching), then the match is legitimate. In other words, if we match, I need to match one of your parents and you need to match one of mine.

It’s important to compare your matches’ DNA to generationally older direct family members such as parents or grandparents, if that’s possible. If your grandparents are available, it’s possible to phase your matches back another generation.

Automatic Phasing at FamilyTreeDNA

FamilyTreeDNA automatically phases your matches to your parents if you test that parent, create or upload a GEDCOM file, and link your test and theirs to your tree in the proper places.

FamilyTreeDNA‘s Family Matching assigns or “buckets” your matches maternally and paternally. Matches are assigned as maternal or paternal matches if one or both parents have tested.

Additionally, FamilyTreeDNA uses triangulated matches from other linked relatives within your tree even if your parents have not tested. If you don’t have your parents, the more people you identify and link to your tree in the proper place, the more people will be assigned to maternal and paternal buckets. FamilyTreeDNA is the only vendor that does this. I wrote about this process in the article, Triangulation in Action at Family Tree DNA.

Parental Phasing Caveats

There are very rare instances where parental phasing may be technically accurate, but not genealogically relevant. By this, I mean that a parent may actually match one of your matches due to endogamy or a population level match, even if it’s considered a false positive because it’s not relevant in a genealogical timeframe.

Conversely, a parent may not match when the segment is actually legitimate, but it’s quite rare and only when pedigree collapse has occurred in a very specific set of circumstances where both parents share a common ancestor.

Let’s take a look at that.

Pedigree Collapse

It’s not terribly uncommon in the not-too-distant past to find first cousins marrying each other, especially in rather closely-knit religious communities. I encounter this in Brethren, Mennonite and Amish families often where the community was small and out-marrying was frowned upon and highly discouraged. These families and sometimes entire church congregations migrated cross-country together for generations.

When pedigree collapse is present, meaning the mother and father share a common ancestor not far in the past, it is possible to inherit half of one segment from Mom and the other half from Dad where those halves originated with the same ancestral couple.

For example, let’s say the matching segment between you and your match is 12 cM in length, shown below. You inherited the blue segment from your Dad and the neighboring peach segment from Mom – shown just below the segment numbers. You received 6 cM from both parents.

Another person’s DNA does match you, shown in the bottom row, but they are not shown on the DNA match list of either of your parents. That’s because the DNA segments of the parents just happened to recombine in 6 cM pieces, respectively, which is below the 7 cM matching threshold of the vendor in this example.

If the person matched you at 12 cM where you inherited 8 cM from one parent and 4 from the other, that person would show on one parent’s match list, but not the other. They would not be on the parent’s match list who contributed only 4 cM simply because the DNA divided and recombined in that manner. They would match you on a longer segment than they match your parent at 8 cM which you might notice as “odd.”

Let’s look at another example.

click to enlarge image

If the matching segment is 20 cM, the person will match you and both of your parents on different pieces of the same segment, given that both segments are above 7 cM. In this case, your match who matches you at 20 cM will match each of your parents at 10 cM.

You would be able to tell that the end location of Dad’s segment is the same as the start location of Mom’s segment.

This is NOT common and is NOT the “go to” answer when you think someone “should” match your parent and does not. It may be worth considering in known pedigree collapse situations.

You can see why someone observing this phenomenon could “presume” that DNA skipped a generation because the person matches you on segments where they don’t match your parent. But DNA didn’t skip anything at all. This circumstance was caused by a combination of pedigree collapse, random division of DNA, then random recombination in the same location where that same DNA segment was divided earlier. Clearly, this sequence of events is not something that happens often.

If you’ve uploaded your DNA to GEDmatch, you can select the “Are your parents related?” function which scans your DNA file for runs of homozygosity (ROH) where your DNA is exactly the same in both parental locations for a significant distance. This suggests that because you inherited the exact same sequence from both parents, that your parents share an ancestor.

If your parents didn’t inherit the same segment of DNA from both parents, or the segment is too short, then they won’t show as “being related,” even if they do share a common ancestor.

Now, let’s look at the opposite situation. Parental phasing and ROH sometimes do occur when common ancestors are far back in time and the match is not genealogically relevant.

Endogamy

I often see non-genealogical matching occur when dealing with endogamy. Endogamy occurs when an entire population has been isolated genetically for a long time. In this circumstance, a substantial part of the population shares common DNA segments because there were few original population founders. Much of the present-day population carries that same DNA. Many people within that population would match on that segment. Think about the Jewish community and indigenous Americans.

Consider our original example, but this time where much of the endogamous population carries all As in these positions because one of the original founders carried that nucleotide sequence. Many people would match lots of other people regardless of whether they are a close relative or share a distant ancestor.

People with endogamous lines do share relatives, but that matching DNA segment originated in ancestors much further back in time. When dealing with endogamy, I use parental phasing as a first step, if possible, then focus on larger matches, generally 20 cM or greater. Smaller matches either aren’t relevant or you often can’t tell if/how they are.

At FamilyTreeDNA, people with endogamy will find many people bucketed on the “Both” tab meaning they triangulate with people linked on both sides of the tester’s tree.

An example of a Jewish person’s bucketed matches based on triangulation with relatives linked in their tree is shown above.

Your siblings, their children, and your children will be related on both your mother’s and father’s sides, but other people typically won’t be unless you have experienced either pedigree collapse where you are related both maternally and paternally through the same ancestors or you descend from an endogamous population.

How Many Identical-by-Chance Matches Will I Have?

If you have both parents available to test, and you’re not dealing with either pedigree collapse or endogamy, you’ll likely find that about 15-20% of your matches don’t match your parents on the same segment and are identical by chance.

With endogamy, you’ll have MANY more matches on your endogamous lines and you’ll have some irrelevant matches, often referred to as “false positive” matches even though they technically aren’t, even using parental phasing.

Your Parents Have DNA That You Don’t

Sometimes people are confused when reviewing their matches and their parent’s match to the same person, especially when they match someone and their parent matches them on a different or an additional segment.

If you match someone on a specific segment and your parents do not, that’s a false positive FOR THAT SEGMENT. Every segment has its own individual history and should be evaluated individually. You can match someone on two segments, one from each parent. Or three segments, one from each parent and one that’s identical by chance. Don’t assume.

Often, your match will match both you and your parent on the same segment – which is a legitimate parentally phased match.

But what if your match matches your parent on a different segment where they don’t match you? That’s a false positive match for you.

Keep in mind that it is possible for one of your matches to match your parent on a separate or an additional segment that IS legitimate. You simply didn’t inherit that particular segment from your parent.

That’s NOT the same situation as someone matching you that does NOT match one of your parents on the same segment – which is an identical by chance or false match.

Your parent having a match that does not match you is the reverse situation.

I have several situations where I match someone on one segment, and they match my parent on the same segment. Additionally, that person matches my parent on another segment that I did NOT inherit from that parent. That’s perfectly normal.

Remember, you only inherit half of your parent’s DNA, so you literally did NOT inherit the other half of their DNA. Your mother, for example, should have twice as many matches as you on her side because roughly half of her matches won’t match you.

That’s exactly why testing your parents and close family members is so critical. Their matches are as valid and relevant to your genealogy as your own. The same is true for other relatives, such as aunts and uncles with whom you share ALL of the same ancestors.

You need to work with your family member’s matches that you don’t share.

No DNA Match Doesn’t Mean You’re Not Related

Some people think that not matching someone on a DNA test is equivalent to saying they aren’t related. Not sharing DNA doesn’t mean you’re not related.

People are often disappointed when they don’t match someone they think they should and interpret that to mean that the testing company is telling them they “aren’t related.” They are upset and take issue with this characterization. But that’s not what it means.

Let’s analyze this a bit further.

First, not sharing DNA with a second cousin once removed (2C1R) or more distant does NOT mean you’re NOT related to that person. It simply means you don’t share any measurable DNA ABOVE THE VENDOR THRESHOLD.

All known second cousins match, but about 10% of third cousins don’t match, and so forth on up the line with each generation further back in time having fewer cousins that match each other.

If you have tested close relatives, check to see if that cousin matches your relatives.

Second, it’s possible to match through the “other” or unexpected parent. I certainly didn’t think this would be the case in my family, because my father is from Appalachia and my mother’s family is primarily from the Netherlands, Germany, Canada, and New England. But I was wrong.

All it took was one German son that settled in Appalachia, and voila, a match through my mother that I surely thought should have been through my father’s side. I have my mother’s DNA and sure enough, my match that I thought should be on my father’s side matches Mom on the same segment where they match me, along with several triangulated matches. Further research confirmed why.

I’ve also encountered situations where I legitimately match someone on both my mother’s and father’s side, on different segments.

Third, imputation can be important for people who don’t match and think they should. Imputation can also cause matching segment length to be overreported.

Ok, so what’s imputation and why do I care?

Imputation

Every DNA vendor today has to use some type of imputation.

Let me explain, in general, what imputation is and why vendors use it.

Over the years, DNA processing vendors who sell DNA chips to testing companies have changed their DNA chips pretty substantially. While genealogical autosomal tests test about 700,000 DNA locations, plus or minus, those locations have changed over time. Today, some of these chips only have 100,000 or so chip locations in common with chips either currently or previously utilized by other vendors.

The vendors who do NOT accept uploads, such as 23andMe or Ancestry, have to develop methods to make their newest customers on their DNA processing vendor’s latest chip compatible with their first customer who was tested on their oldest chip – and all iterations in-between.

Vendors who do accept transfers/uploads from other vendors have to equalize any number of vendors’ chips when their customers upload those files.

Imputation is the scientific way to achieve this cross-platform functionality and has been widely used in the industry since 2017.

Imputation, in essence, fills in the blanks between tested locations with the “most likely” DNA found in the human population based on what’s surrounding the blank location.

Think of the word C_T. There are a limited number of letters and words that are candidates for C_T. If you use the word in a sentence, your odds of accuracy increase dramatically. Think of a genetic string of nucleotides as a sentence.

Imputation can be incorrect and can cause both false positive and false negative matches.

For the most part, imputation does not affect close family matches as much as more distant matches. In other words, imputation is NOT going to cause close family members not to match.

Imputation may cause more distant family members not to match, or to have a false positive match when imputation is incorrect.

Imputation is actually MUCH less problematic than I initially expected.

The most likely effect of imputation is to cause a match to be just above or below the vendor threshold.

How can we minimize the effects of imputation?

  • Generally, the best result will be achieved if both people test at the same vendor where their DNA is processed on the same chip and less imputation is required.
  • Upload the results of both people to both MyHeritage and FamilyTreeDNA. If your match results are generally consistent at those vendors, imputation is not a factor.
  • GEDmatch does not use imputation but attempts to overcome files with low overlapping regions by allowing larger mismatch areas. I find their matches to be less accurate than at the various vendors.

Additionally, Ancestry has a few complicating factors.

Ancestry Issues

AncestryDNA is different in three ways.

  • Ancestry doesn’t provide segment information so it’s impossible to triangulate or identify the segment or chromosome where people match. There is no chromosome browser or triangulation tool.
  • Ancestry down-weights and removes some segments in areas where they feel that people are “too matchy.” You can read Ancestry’s white papers here and here.

These “personal pileup regions,” as they are known, can be important genealogically. In my case, these are my mother’s Acadian ancestors. Yes, this is an endogamous population and also suffers from pedigree collapse, but since this is only one of my mother’s great-grandparents, this match information is useful and should not be removed.

  • Ancestry doesn’t show matches in common if the shared segments are less than 20cM. Therefore, you may not see someone on a shared match list with a relative when they actually are a shared match.

If two people both match a third person on less than a 20 cM segment at Ancestry, the third person won’t appear on the other person’s shared match list. So, if I match John Doe on 19 cM of DNA, and I looked at the shared matches with my Dad, John Doe does NOT appear on the shared match list of me and my Dad – even though he is a match to both of us at 19 cM.

The only way to determine if John Doe is a shared match is to check my Dad’s and my match list individually, which means Dad and I will need to individually search for John Doe.

Caveat here – Ancestry’s search sometimes does not work correctly.

Might someone who doesn’t understand that the shared match list doesn’t show everyone who shares DNA with both people presume that the ancestral DNA of that ancestor “skipped a generation” because John Doe matches me with a known ancestor, and not Dad on our shared match list? I mean, wouldn’t you think that a shared match would be shown on a tab labeled “Shared Matches,” especially since there is no disclaimer?

Yes, people can be forgiven for believing that somehow DNA “skipped” a generation in this circumstance, especially if they are relatively inexperienced and they don’t understand Ancestry’s anomalies or know that they need to or how to search for matches individually.

Even if John Doe does match me and Dad both, we still need to confirm that it’s on the same segment AND it’s a legitimate match, not IBC. You can’t perform either of these functions at Ancestry, but you can elsewhere.

Ancestry WorkArounds

To obtain this functionality, people can upload their DNA files for free to both FamilyTreeDNA and MyHeritage, companies that do provide full shared DNA reporting (in common with) lists of ALL matches and do provide segment information with chromosome browsers. Furthermore, both provide triangulation in different ways.

Matching is free, but an inexpensive unlock is required at both vendors to access advanced tools such as Family Matching (bucketing) and triangulation at Family Tree DNA and phasing/triangulation at MyHeritage.

I wrote about Triangulation in Action at FamilyTreeDNA, here.

MyHeritage actually brackets triangulated segments for customers on their chromosome browser, including parents, so you get triangulation and parental phasing at the same time if you and your parent have both tested or uploaded your DNA file to MyHeritage. You can upload, for free, here.

In this example, my mother is matching to me in red on the entire length of chromosome 18, of course, and three other maternal cousins triangulate with me and mother inside the bracketed portion of chromosome 18. Please note that if any one of the people included in the chromosome browser comparison do not triangulate, no bracket is drawn around any others who do triangulate. It’s all or nothing. I remove people one by one to see if people triangulate – or build one by one with my mother included.

I wrote about Triangulation in Action at MyHeritage, here.

People can also upload to GEDmatch, a third-party site. While GEDmatch is less reliable for matching, you can adjust your search thresholds which you cannot do at other vendors. I don’t recommend routinely working below 7 cM. I occasionally use GEDmatch to see if a pedigree collapse segment has recombined below another vendor’s segment matching threshold.

Do NOT check the box to prevent hard breaks when selecting the One-to-One comparison. Checking that box allows GEDmatch to combine smaller matching segments into mega-segments for matching.

I wrote about Triangulation in Action at GEDmatch, here.

Transferring/Uploading Your DNA 

If you want to transfer your DNA to one of these vendors, you must download the DNA file from one vendor and upload it to another. That process does NOT remove your DNA file from the vendor where you tested, unless you select that option entirely separately.

I wrote full step-by-step transfer/upload instructions for each vendor, here.

Testing Close Relatives Is VERY Useful – Just Not for Triangulation

Of course, your best bet if you don’t have your parents available to test is to test as many of your grandparents, great-aunts/uncles, aunts, and uncles as possible. Test your siblings as well, because they will have inherited some of the same and some different segments of DNA from your parents – which means they carry different pieces of your ancestors’ DNA.

Just because close relatives don’t make good triangulation candidates doesn’t mean they aren’t valuable. Close relatives are golden because when they DO share a match with you, you know where to start looking for a common ancestor, even if your relative matches that person on a different segment than you do.

Close relatives are also important because they will share pieces of your common ancestor’s DNA that you don’t. Their matches can unlock the answers to your genealogy questions.

Ok, back to triangulation.

Triangulated Matches

A triangulated match is, of course, when three people all descended from a common ancestor and match each other on the same segment of DNA.

That means all three people’s DNA matches each other on that same segment, confirming that the match is not by chance, and that segment did descend from a common ancestor or ancestral couple.

But, is this always true? You’re going to hate this answer…

“It depends.”

You knew that was coming, didn’t you! 😊

It depends on the circumstances and relationships of the three people involved.

  • One of those three people can match the other two by chance, not by descent, especially if two of those people are close relatives to each other.
  • Identical by chance means that one of you didn’t inherit that DNA from one single parent. That zigzag phenomenon.
  • Furthermore, triangulated DNA is only valid as far back as the closest common ancestor of any two of the three people.

Let’s explore some examples.

Building Triangulation Evidence – Ingredients and a Recipe

The strongest case of triangulation is when:

  • You and at least two additional cousins match on the same segment AND
  • Descend through different children of the common ancestral couple

Let’s look at a valid triangulated match.

In this first example, the magenta segment of DNA is at least partially shared by four of the six cousins and triangulates to their common great-grandfather. Let’s say that these cousins then match with two other people descended from different children of their great-great-great-grandparents on this same segment. Then the entire triangulation group will have confirmed that segment’s origin and push the descent of that segment back another two generations.

These people all coalesce into one line with their common great-grandparents.

I’m only showing 3 generations in this triangulated match, but the concept is the same no matter how many generations you reach back in time. Although, over time, segments inherited from any specific ancestor become smaller and smaller until they are no longer passed to the next generation.

In this pedigree chart, we’re only tracking the magenta DNA which is passed generation to generation in descendants.

Eventually, of course, those segments become smaller and indistinguishable as they either aren’t passed on at all or drop below vendor matching thresholds.

This chart shows the average amount of DNA you would carry from each generational ancestor. You inherit half of each parent’s DNA, but back further than that, you don’t receive exactly half of any ancestor’s DNA in any generation. Larger segments are generally cut in two and passed on partially, but smaller segments are often either passed on whole or not at all.

On average, you’ll carry 7 cM of your eight-times-great-grandparents. In reality, you may carry more or you may not carry any – and you are unlikely to carry the same segment as any random other descendants but we know it happens and you’ll find them if enough (or the right) descendants test.

Putting this another way, if you divide all of your approximate 7000 cM of DNA into 7 cM segments of equal length – you’ll have 1000 7 cM segments. So will every other descendant of your eight-times-great-grandparent. You can see how small the chances are of you both inheriting that same exact 7 cM segment through ten inheritance/transmission events, each. Yet it does happen.

I have several triangulated matches with descendants of Charles Dodson and his wife, Anne through multiple of their 9 (or so) children, ten generations back in my tree. Those triangulated matches range from 7-38 cM. It’s possible that those three largest matches at 38 cM could be related through multiple ancestors because we all have holes in our trees – including Anne’s surname.

Click to enlarge image

It helps immensely that Charles Dodson had several children who were quite prolific as well.

Of course, the further back in time, the more “proof” is necessary to eliminate other unknown common ancestors. This is exactly why matching through different children is important for triangulation and ancestor confirmation.

The method we use to confirm the common ancestor is that all of the descendants who match the tester on the same segment all also match each other. This greatly reduces the chances that these people are matching by chance. The more people in the triangulation group, the stronger the evidence. Of course, parental phasing or cross-matching, where available is an added confirmation bonus.

In our magenta inheritance example, we saw that three of the males and one of the females from three different descendants of the great-grandparents all carry at least a portion of that magenta segment of great-grandpa’s DNA.

Now, let’s take a look at a different scenario.

Why can’t siblings or close relatives be used as two of the three people needed for triangulation?

Aunts and Uncles

We know that the best way to determine if a match is valid is by parental phasing – your match also matching to one of your parents.

If both parents aren’t available, looking for close family matches in common with your match is the next hint that genealogists seek.

Let’s say that you and your match both match your aunt or uncle in common or their children.

You and your aunts or uncles matching DNA only pushes your common ancestor back to your grandparents.

At that point, your match is in essence matching to a segment that belongs to your grandparents. Your matches’ DNA, or your grandparents’ DNA could have randomly recombined and you and your aunt/cousins could be matching that third person by chance.

Ok, then, what about siblings?

Siblings

The most recent common ancestor (MRCA) of you and someone who also matches your sibling is your parents. Therefore, you and your sibling actually only count as one “person” in this scenario. In essence, it’s the DNA of your parent(s) that is matching that third person, so it’s not true triangulation. It’s the same situation as above with aunts/uncles, except the common ancestor is closer than your grandparents.

The DNA of your parents could have recombined in both siblings to look like a match to your match’s family. Or vice versa. Remember Parental Cross-Matching.

If you and a sibling inherited EXACTLY the same segment of your Mom’s and Dad’s DNA, and you match someone by chance – that person will match your sibling by chance as well.

In this example, you can see that both siblings 1 and 2 inherited the exact same segments of DNA at the same locations from both of their parents.

Of course, they also inherited segments at different locations that we’re not looking at that won’t match exactly between siblings, unless they are identical twins. But in this case, the inherited segments of both siblings will match someone whose DNA randomly combined with green or magenta dots in these positions to match a cross-section of both parents.

How False Positives Work and How to Avoid Them

We saw in our first example, displayed again above, what a valid triangulated match looks like. Now let’s expand this view and take a look more specifically at how false positive matches occur.

On the left-hand (blue) side of this graphic, we see four siblings that descend through their father from Great-grandpa who contributed that large magenta segment of DNA. That segment becomes reduced in descendants in subsequent generations.

In downstream generations, we can see gold, white and green segments being added to the DNA inherited by the four children from their ancestor’s spouses. Dad’s DNA is shown on the left side of each child, and Mom’s on the right.

  • Blue Children 1 and 2 inherited the same segments of DNA from Mom and Dad. Magenta from Dad and green from Mom.
  • Blue Child 3 inherited two magenta segments from Dad in positions 1 and 2 and one gold segment from Dad in position 3. They inherited all white segments from Mom.
  • Blue Child 4 inherited all gold segments from Dad and all white segments from Mom.

The family on the blue left-hand side is NOT related to the pink family shown at right. That’s important to remember.

I’ve intentionally constructed this graphic so that you can see several identical by chance (IBC) matches.

Child 5, the first pink sibling carries a white segment in position 1 from Dad and gold segments in positions 2 and 3 from Dad. From Mom, they inherited a green segment in position 1, magenta in position 2 and green in position 3.

IBC Match 1 – Looking at the blue siblings, we see that based on the DNA inherited from Pink Child 5’s parents, Pink Child 5 matches Blue Child 4 with white, gold and gold in positions 1-3, even though they weren’t inherited from the same parent in Blue Child 4. I circled this match in blue.

IBC Match 2 – Pink Child 5 also matches Blue Children 1 and 2 (red circles) because Pink Child 5 has green, magenta, and green in positions 1-3 and so do Blue Children 1 and 2. However, Blue Children 1 and 2 inherited the green and magenta segments from Mom and Dad respectively, not just from one parent.

Pink Child 5 matches Blue Children 1, 2 and 4, but not because they match by descent, but because their DNA zigzags back and forth between the blue children’s DNA contributed by both parents.

Therefore, while Pink Child 5 matches three of the Blue Children, they do not match either parent of the Blue Children.

IBC Match 3 – Pink Child 6 matches Blue Child 3 with white, magenta and gold in positions 1-3 based on the same colors of dots in those same positions found in Blue Child 3 – but inherited both paternally and maternally.

You can see that if we had the four parents available to test, that none of the Pink Children would match either the Blue Children’s mother or father and none of the Blue Children would match either of the Pink Children’s mother or father.

This is why we can’t use either siblings or close family relatives for triangulation.

Distant Cousins Are Best for Triangulation & Here’s Why

When triangulating with 3 people, the most recent common ancestor (MRCA) intersection of the closest two people is the place at which triangulation turns into only two lines being compared and ceases being triangulation. Triangle means 3.

If siblings are 2 of the 3 matching people, then their parents are essentially being compared to the third person.

If you, your aunt/uncle, and a third person match, your grandparents are the place in your tree where three lines converge into two.

The same holds true if you’re matching against a sibling pair on your match’s side, or a match and their aunt/uncle, etc.

The further back in your tree you can push that MRCA intersection, the more your triangulated match provides confirming evidence of a common ancestor and that the match is valid and not caused by random recombination.

That’s exactly what the descendants of Charles Dodson have been able to do through triangulation with multiple descendants from several of his children.

It’s also worth mentioning at this point that the reason autosomal DNA testing uses hundreds/thousands of base pairs in a comparison window and not 3 or 6 dots like in my example is that the probability of longer segments of DNA simply randomly matching by chance is reduced with length and SNP density which is the number of SNP locations tested within that cM range.

Hence a 7 cM/500 SNP minimum is the combined rule of thumb. At that level, roughly half of your matches will be valid and half will be identical by chance unless you’re dealing with endogamy. Then, raise your threshold accordingly.

Ok, So Where are We? A Triangulation Checklist for You!

I know this has been a relatively long educational article, but it’s important to really understand that testing close relatives is VERY important, but also why we can’t effectively use them for triangulation.

Here’s a handy-dandy summary matching/triangulation checklist for you to use as you work through your matches.

  • You inherit half of each of your parents’ DNA. There is no other place for you to obtain or inherit your DNA. There is no DNA fairy sprinkling you with DNA from another source:)
  • DNA does NOT skip generations, although in occasional rare circumstances, it may appear that this happened. In this situation, it’s incumbent upon you, the genealogist, to PROVE that an exception has occurred if you really believe it has. Those circumstances might be pedigree collapse or perhaps imputation. You’ll need to compare matches at vendors who provide a chromosome browser, triangulation, and full shared match list information. Never assume that you are the exception without hard and fast proof. We all know about assume, right?
  • Your siblings inherit half of your parents’ DNA too, but not the same exact half of your parent’s DNA that you other siblings did (unless they are identical twins.) You may inherit the exact same DNA from either or both of your parents on certain segments.
  • Your matches may match your parents on different or an additional segment that you did not inherit.
  • Every segment has an individual history. Evaluate every matching segment separately. One matching segment with someone could be maternal, one paternal, and one identical by chance.
  • You can confirm matches as valid if your match matches one of your parents, and you match one of your match’s parents. Parental Phasing is when your match matches your parent. Parental Cross-Matching is when you both match one of each other’s parents. To be complete, both people who match each other need to match one of the parents of the other person. This rule still holds even if you have a known common ancestor. I can’t even begin to tell you how many times I’ve been fooled.
  • 15-20% (or more with endogamy) of your matches will be identical by chance because either your DNA or your match’s DNA aligns in such a way that while they match you, they don’t match either of your parents.
  • Your siblings, aunts, and uncles will often inherit the same DNA as you – which means that identical by chance matches will also match them. That’s why we don’t use close family members for triangulation. We do utilize close family members to generate common match hints. (Remember the 20 cM shared match caveat at Ancestry)
  • While your siblings, aunts, and uncles are too close to use for triangulation, they are wonderful to identify ancestral matches. Some of their matches will match you as well, and some will not because your close family members inherited segments of your ancestor’s DNA that you did not. Everyone should test their oldest family members.
  • Triangulate your close family member’s matches separately from your own to shed more light on your ancestors.
  • Endogamy may interfere with parental phasing, meaning you may match because you and/or your match may have inherited some of the same DNA segment(s) from both sides of your tree and/or more DNA than might otherwise be expected.
  • Pedigree collapse needs to be considered when using parental phasing, especially when the same ancestor appears on both sides of your family tree. You may share more DNA with a match than expected.
  • Conversely, with pedigree collapse, your match may not match your parents, or vice versa, if a segment happens to have recombined in you in a way that drops the matching segments of your parents beneath the vendor’s match threshold.
  • While you will match all of your second cousins, you will only match approximately 90% of your third cousins and proportionally fewer as your relationship reaches further back in time.
  • Not being a DNA match with someone does NOT mean you’re NOT related to them, unless of course, you’re a second cousin (2C) or closer. It simply means you don’t carry any common ancestral segments above vendor thresholds.
  • At 2C or closer, if you’re not a DNA match, other alternative situations need to be considered – including the transfer/upload of the wrong person’s DNA file.
  • Imputation, a scientific process required of vendors may interfere with matching, especially in more distant relatives who have tested on different platforms.
  • Imputation artifacts will be less obvious when people are more closely related, meaning closer relatives can be expected to match on more and larger segments and imputation errors make less difference.
  • Imputation will not cause close relatives, meaning 2C or closer, to not match each other.
  • In addition to not supporting segment matching information, Ancestry down-weights some segments, removes some matching DNA, and does not show shared matches below 20cM, causing some people to misinterpret their lack of common matches in various ways.
  • To resolve questions about matching issues at Ancestry, testers can transfer/upload their DNA files to MyHeritage, FamilyTreeDNA, and GEDmatch and look for consistent matches on the same segment. Start and end locations may vary to some extent between vendors, but the segment size should be basically in the same location and roughly the same size.
  • GEDmatch does not use imputation but allows larger non-matching segments to combine as a single segment which sometimes causes extremely “generous” matches. GEDmatch matching is less reliable than FamilyTreeDNA or MyHeritage, but you can adjust the matching thresholds.
  • The best situation for matching is for both people to test at the same vendor who supports and provides segment data and a chromosome browser such as 23andMe, FamilyTreeDNA, or MyHeritage.
  • Siblings cannot be used for triangulation because the most recent common ancestor (MRCA) between you and your siblings is your parents. Therefore, the “three” people in the triangulation group is reduced to two lines immediately.
  • Uncles and aunts should not be used for triangulation because the most recent common ancestors between you and your aunts and uncles are your grandparents.
  • Conversely, you should not consider triangulating with siblings and close family members of your matches as proof of an ancestral relationship.
  • A triangulation group of 3 people is only confirmation as far back as when two of those people’s lines converge and reach a common ancestor.
  • Identical by chance (IBC) matching occurs when DNA from the maternal and paternal sides are mixed positionally in the child to resemble a maternal/paternal side match with someone else.
  • Identical by chance DNA admixture (when compared to a match) could have occurred in your parents or grandparent’s generation, or earlier, so the further back in time that people in a triangulation group reach, the more reliable the triangulation group is likely to be.
  • The larger the segments and/or the triangulation group, the stronger the evidence for a specific confirmed common ancestor.
  • Early families with a very large number of descendants may have many matching and triangulated members, even 9 or 10 generations later.
  • While exactly 50% of each ancestor’s DNA is not passed in each generation, on average, you will carry 7 cM of your ancestors 10 generations back in your tree. However, you may carry more, or none.
  • The percentage of matching descendants decreases with each generation beyond great-grandparents.
  • The ideal situation for triangulation is a significant number of people, greater than three, who match on the same reasonably sized segment (7 cM/500 SNP or larger) and descend from the same ancestor (or ancestral couple) through different children whose spouses in descendant generations are not also related.
  • This means that tree completion is an important factor in match/triangulation reliability.
  • Triangulating through different children of the ancestral couple makes it significantly less likely that a different unknown common ancestor is contributing that segment of DNA – like an unknown wife in a descendant generation.

Whew!!!

The Bottom Line

Here’s the bottom line.

  1. Don’t use close relatives to triangulate.
  2. Use parents for Parental Phasing.
  3. Use Parental Cross-Matching when possible.
  4. Use close relatives to look for shared common matches that may lead to triangulation possibilities.
  5. Triangulate your close relatives’ DNA in addition to your own for bonus genealogical information. They will match people that you don’t.
  6. For the most reliable triangulation results, use the most distant relatives possible, descended through different children of the common ancestral couple.
  7. Keep this checklist of best practices, cautions, and caveats handy and check the list as necessary when evaluating the strength of any match or triangulation group. It serves as a good reminder for what to check if something seems “off” or unusual.

Feel free to share and pass this article (and checklist) on to your genealogy buddies and matches as you explain triangulation and collaborate on your genealogy.

Have fun!!!

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Concepts – Segment Size, Legitimate and False Matches

Matchmaker, matchmaker, make me a match!

One of the questions I often receive about autosomal DNA is, “What, EXACTLY, is a match?”  The answer at first glance seems evident, meaning when you and someone else are shown on each other’s match lists, but it really isn’t that simple.

What I’d like to discuss today is what actually constitutes a match – and the difference between legitimate or real matches and false matches, also called false positives.

Let’s look at a few definitions before we go any further.

Definitions

  • A Match – when you and another person are found on each other’s match lists at a testing vendor. You may match that person on one or more segments of DNA.
  • Matching Segment – when a particular segment of DNA on a particular chromosome matches to another person. You may have multiple segment matches with someone, if they are closely related, or only one segment match if they are more distantly related.
  • False Match – also known as a false positive match. This occurs when you match someone that is not identical by descent (IBD), but identical by chance (IBC), meaning that your DNA and theirs just happened to match, as a happenstance function of your mother and father’s DNA aligning in such a way that you match the other person, but neither your mother or father match that person on that segment.
  • Legitimate Match – meaning a match that is a result of the DNA that you inherited from one of your parents. This is the opposite of a false positive match.  Legitimate matches are identical by descent (IBD.)  Some IBD matches are considered to be identical by population, (IBP) because they are a result of a particular DNA segment being present in a significant portion of a given population from which you and your match both descend. Ideally, legitimate matches are not IBP and are instead indicative of a more recent genealogical ancestor that can (potentially) be identified.

You can read about Identical by Descent and Identical by Chance here.

  • Endogamy – an occurrence in which people intermarry repeatedly with others in a closed community, effectively passing the same DNA around and around in descendants without introducing different/new DNA from non-related individuals. People from endogamous communities, such as Jewish and Amish groups, will share more DNA and more small segments of DNA than people who are not from endogamous communities.  Fully endogamous individuals have about three times as many autosomal matches as non-endogamous individuals.
  • False Negative Match – a situation where someone doesn’t match that should. False negatives are very difficult to discern.  We most often see them when a match is hovering at a match threshold and by lowing the threshold slightly, the match is then exposed.  False negative segments can sometimes be detected when comparing DNA of close relatives and can be caused by read errors that break a segment in two, resulting in two segments that are too small to be reported individually as a match.  False negatives can also be caused by population phasing which strips out segments that are deemed to be “too matchy” by Ancestry’s Timber algorithm.
  • Parental or Family Phasing – utilizing the DNA of your parents or other close family members to determine which side of the family a match derives from. Actual phasing means to determine which parts of your DNA come from which parent by comparing your DNA to at least one, if not both parents.  The results of phasing are that we can identify matches to family groups such as the Phased Family Finder results at Family Tree DNA that designate matches as maternal or paternal based on phased results for you and family members, up to third cousins.
  • Population Based Phasing – In another context, phasing can refer to academic phasing where some DNA that is population based is removed from an individual’s results before matching to others. Ancestry does this with their Timber program, effectively segmenting results and sometimes removing valid IBD segments.  This is not the type of phasing that we will be referring to in this article and parental/family phasing should not be confused with population/academic phasing.

IBD and IBC Match Examples

It’s important to understand the definitions of Identical by Descent and Identical by Chance.

I’ve created some easy examples.

Let’s say that a match is defined as any 10 DNA locations in a row that match.  To keep this comparison simple, I’m only showing 10 locations.

In the examples below, you are the first person, on the left, and your DNA strands are showing.  You have a pink strand that you inherited from Mom and a blue strand inherited from Dad.  Mom’s 10 locations are all filled with A and Dad’s locations are all filled with T.  Unfortunately, Mother Nature doesn’t keep your Mom’s and Dad’s strands on one side or the other, so their DNA is mixed together in you.  In other words, you can’t tell which parts of your DNA are whose.  However, for our example, we’re keeping them separate because it’s easier to understand that way.

Legitimate Match – Identical by Descent from Mother

matches-ibd-mom

In the example above, Person B, your match, has all As.  They will match you and your mother, both, meaning the match between you and person B is identical by descent.  This means you match them because you inherited the matching DNA from your mother. The matching DNA is bordered in black.

Legitimate Match – Identical by Descent from Father

In this second example, Person C has all T’s and matches both you and your Dad, meaning the match is identical by descent from your father’s side.

matches-ibd-dad

You can clearly see that you can have two different people match you on the same exact segment location, but not match each other.  Person B and Person C both match you on the same location, but they very clearly do not match each other because Person B carries your mother’s DNA and Person C carries your father’s DNA.  These three people (you, Person B and Person C) do NOT triangulate, because B and C do not match each other.  The article, “Concepts – Match Groups and Triangulation” provides more details on triangulation.

Triangulation is how we prove that individuals descend from a common ancestor.

If Person B and Person C both descended from your mother’s side and matched you, then they would both carry all As in those locations, and they would match you, your mother and each other.  In this case, they would triangulate with you and your mother.

False Positive or Identical by Chance Match

This third example shows that Person D does technically match you, because they have all As and Ts, but they match you by zigzagging back and forth between your Mom’s and Dad’s DNA strands.  Of course, there is no way for you to know this without matching Person D against both of your parents to see if they match either parent.  If your match does not match either parent, the match is a false positive, meaning it is not a legitimate match.  The match is identical by chance (IBC.)

matches-ibc

One clue as to whether a match is IBC or IBD, even without your parents, is whether the person matches you and other close relatives on this same segment.  If not, then the match may be IBC. If the match also matches close relatives on this segment, then the match is very likely IBD.  Of course, the segment size matters too, which we’ll discuss momentarily.

If a person triangulates with 2 or more relatives who descend from the same ancestor, then the match is identical by descent, and not identical by chance.

False Negative Match

This last example shows a false negative.  The DNA of Person E had a read error at location 5, meaning that there are not 10 locations in a row that match.  This causes you and Person E to NOT be shown as a match, creating a false negative situation, because you actually do match if Person E hadn’t had the read error.

matches-false-negative

Of course, false negatives are by definition very hard to identify, because you can’t see them.

Comparisons to Your Parents

Legitimate matches will phase to your parents – meaning that you will match Person B on the same amount of a specific segment, or a smaller portion of that segment, as one of your parents.

False matches mean that you match the person, but neither of your parents matches that person, meaning that the segment in question is identical by chance, not by descent.

Comparing your matches to both of your parents is the easiest litmus paper test of whether your matches are legitimate or not.  Of course, the caveat is that you must have both of your parents available to fully phase your results.

Many of us don’t have both parents available to test, so let’s take a look at how often false positive matches really do occur.

False Positive Matches

How often do false matches really happen?

The answer to that question depends on the size of the segments you are comparing.

Very small segments, say at 1cM, are very likely to match randomly, because they are so small.  You can read more about SNPs and centiMorgans (cM) here.

As a rule of thumb, the larger the matching segment as measured in cM, with more SNPs in that segment:

  • The stronger the match is considered to be
  • The more likely the match is to be IBD and not IBC
  • The closer in time the common ancestor, facilitating the identification of said ancestor

Just in case we forget sometimes, identifying ancestors IS the purpose of genetic genealogy, although it seems like we sometimes get all geeked out by the science itself and process of matching!  (I can hear you thinking, “speak for yourself, Roberta.”)

It’s Just a Phase!!!

Let’s look at an example of phasing a child’s matches against those of their parents.

In our example, we have a non-endogamous female child (so they inherit an X chromosome from both parents) whose matches are being compared to her parents.

I’m utilizing files from Family Tree DNA. Ancestry does not provide segment data, so Ancestry files can’t be used.  At 23andMe, coordinating the security surrounding 3 individuals results and trying to make sure that the child and both parents all have access to the same individuals through sharing would be a nightmare, so the only vendor’s results you can reasonably utilize for phasing is Family Tree DNA.

You can download the matches for each person by chromosome segment by selecting the chromosome browser and the “Download All Matches to Excel (CSV Format)” at the top right above chromosome 1.

matches-chromosomr-browser

All segment matches 1cM and above will be downloaded into a CSV file, which I then save as an Excel spreadsheet.

I downloaded the files for both parents and the child. I deleted segments below 3cM.

About 75% of the rows in the files were segments below 3cM. In part, I deleted these segments due to the sheer size and the fact that the segment matching was a manual process.  In part, I did this because I already knew that segments below 3 cM weren’t terribly useful.

Rows Father Mother Child
Total 26,887 20,395 23,681
< 3 cM removed 20,461 15,025 17,784
Total Processed 6,426 5,370 5,897

Because I have the ability to phase these matches against both parents, I wanted to see how many of the matches in each category were indeed legitimate matches and how many were false positives, meaning identical by chance.

How does one go about doing that, exactly?

Downloading the Files

Let’s talk about how to make this process easy, at least as easy as possible.

Step one is downloading the chromosome browser matches for all 3 individuals, the child and both parents.

First, I downloaded the child’s chromosome browser match file and opened the spreadsheet.

Second, I downloaded the mother’s file, colored all of her rows pink, then appended the mother’s rows into the child’s spreadsheet.

Third, I did the same with the father’s file, coloring his rows blue.

After I had all three files in one spreadsheet, I sorted the columns by segment size and removed the segments below 3cM.

Next, I sorted the remaining items on the spreadsheet, in order, by column, as follows:

  • End
  • Start
  • Chromosome
  • Matchname

matches-both-parents

My resulting spreadsheet looked like this.  Sorting in the order prescribed provides you with the matches to each person in chromosome and segment order, facilitating easy (OK, relatively easy) visual comparison for matching segments.

I then colored all of the child’s NON-matching segments green so that I could see (and eventually filter the matchname column by) the green color indicating that they were NOT matches.  Do this only for the child, or the white (non-colored) rows.  The child’s matchname only gets colored green if there is no corresponding match to a parent for that same person on that same chromosome segment.

matches-child-some-parents

All of the child’s matches that DON’T have a corresponding parent match in pink or blue for that same person on that same segment will be colored green.  I’ve boxed the matches so you can see that they do match, and that they aren’t colored green.

In the above example, Donald and Gaff don’t match either parent, so they are all green.  Mess does match the father on some segments, so those segments are boxed, but the rest of Mess doesn’t match a parent, so is colored green.  Sarah doesn’t match any parent, so she is entirely green.

Yes, you do manually have to go through every row on this combined spreadsheet.

If you’re going to phase your matches against your parent or parents, you’ll want to know what to expect.  Just because you’ve seen one match does not mean you’ve seen them all.

What is a Match?

So, finally, the answer to the original question, “What is a Match?”  Yes, I know this was the long way around the block.

In the exercise above, we weren’t evaluating matches, we were just determining whether or not the child’s match also matched the parent on the same segment, but sometimes it’s not clear whether they do or do not match.

matches-child-mess

In the case of the second match with Mess on chromosome 11, above, the starting and ending locations, and the number of cM and segments are exactly the same, so it’s easy to determine that Mess matches both the child and the father on chromosome 11. All matches aren’t so straightforward.

Typical Match

matches-typical

This looks like your typical match for one person, in this case, Cecelia.  The child (white rows) matches Cecelia on three segments that don’t also match the child’s mother (pink rows.)  Those non-matching child’s rows are colored green in the match column.  The child matches Cecelia on two segments that also match the mother, on chromosome 20 and the X chromosome.  Those matching segments are boxed in black.

The segments in both of these matches have exact overlaps, meaning they start and end in exactly the same location, but that’s not always the case.

And for the record, matches that begin and/or end in the same location are NOT more likely to be legitimate matches than those that start and end in different locations.  Vendors use small buckets for matching, and if you fall into any part of the bucket, even if your match doesn’t entirely fill the bucket, the bucket is considered occupied.  So what you’re seeing are the “fuzzy” bucket boundaries.

(Over)Hanging Chad

matches-overhanging

In this case, Chad’s match overhangs on each end.  You can see that Chad’s match to the child begins at 52,722,923 before the mother’s match at 53,176,407.

At the end location, the child’s matching segment also extends beyond the mother’s, meaning the child matches Chad on a longer segment than the mother.  This means that the segment sections before 53,176,407 and after 61,495,890 are false negative matches, because Chad does not also match the child’s mother of these portions of the segment.

This segment still counts as a match though, because on the majority of the segment, Chad does match both the child and the mother.

Nested Match

matches-nested

This example shows a nested match, where the parent’s match to Randy begins before the child’s and ends after the child’s, meaning that the child’s matching DNA segment to Randy is entirely nested within the mother’s.  In other words, pieces got shaved off of both ends of this segment when the child was inheriting from her mother.

No Common Matches

matches-no-common

Sometimes, the child and the parent will both match the same person, but there are no common segments.  Don’t read more into this than what it is.  The child’s matches to Mary are false matches.  We have no way to judge the mother’s matches, except for segment size probability, which we’ll discuss shortly.

Look Ma, No Parents

matches-no-parents

In this case, the child matches Don on 5 segments, including a reasonably large segment on chromosome 9, but there are no matches between Don and either parent.  I went back and looked at this to be sure I hadn’t missed something.

This could, possibly, be an instance of an unseen a false negative, meaning perhaps there is a read issue in the parent’s file on chromosome 9, precluding a match.  However, in this case, since Family Tree DNA does report matches down to 1cM, it would have to be an awfully large read error for that to occur.  Family Tree DNA does have quality control standards in place and each file must pass the quality threshold to be put into the matching data base.  So, in this case, I doubt that the problem is a false negative.

Just because there are multiple IBC matches to Don doesn’t mean any of those are incorrect.  It’s just the way that the DNA is inherited and it’s why this type of a match is called identical by chance – the key word being chance.

Split Match

matches-split

This split match is very interesting.  If you look closely, you’ll notice that Diane matches Mom on the entire segment on chromosome 12, but the child’s match is broken into two.  However, the number of SNPs adds up to the same, and the number of cM is close.  This suggests that there is a read error in the child’s file forcing the child’s match to Diane into two pieces.

If the segments broken apart were smaller, under the match threshold, and there were no other higher matches on other segments, this match would not be shown and would fall into the False Negative category.  However, since that’s not the case, it’s a legitimate match and just falls into the “interesting” category.

The Deceptive Match

matches-surname

Don’t be fooled by seeing a family name in the match column and deciding it’s a legitimate match.  Harrold is a family surname and Mr. Harrold does not match either of the child’s parents, on any segment.  So not a legitimate match, no matter how much you want it to be!

Suspicious Match – Probably not Real

matches-suspicious

This technically is a match, because part of the DNA that Daryl matches between Mom and the child does overlap, from 111,236,840 to 113,275,838.  However, if you look at the entire match, you’ll notice that not a lot of that segment overlaps, and the number of cMs is already low in the child’s match.  There is no way to calculate the number of cMs and SNPs in the overlapping part of the segment, but suffice it to say that it’s smaller, and probably substantially smaller, than the 3.32 total match for the child.

It’s up to you whether you actually count this as a match or not.  I just hope this isn’t one of those matches you REALLY need.  However, in this case, the Mom’s match at 15.46 cM is 99% likely to be a legitimate match, so you really don’t need the child’s match at all!!!

So, Judge Judy, What’s the Verdict?

How did our parental phasing turn out?  What did we learn?  How many segments matched both the child and a parent, and how many were false matches?

In each cM Size category below, I’ve included the total number of child’s match rows found in that category, the number of parent/child matches, the percent of parent/child matches, the number of matches to the child that did NOT match the parent, and the percent of non-matches. A non-match means a false match.

So, what the verdict?

matches-parent-child-phased-segment-match-chart

It’s interesting to note that we just approach the 50% mark for phased matches in the 7-7.99 cM bracket.

The bracket just beneath that, 6-6.99 shows only a 30% parent/child match rate, as does 5-5.99.  At 3 cM and 4 cM few matches phase to the parents, but some do, and could potentially be useful in groups of people descended from a known common ancestor and in conjunction with larger matches on other segments. Certainly segments at 3 cM and 4 cM alone aren’t very reliable or useful, but that doesn’t mean they couldn’t potentially be used in other contexts, nor are they always wrong. The smaller the segment, the less confidence we can have based on that segment alone, at least below 9-15cM.

Above the 50% match level, we quickly reach the 90th percentile in the 9-9.99 cM bracket, and above 10 cM, we’re virtually assured of a phased match, but not quite 100% of the time.

It isn’t until we reach the 16cM category that we actually reach the 100% bracket, and there is still an outlier found in the 18-18.99 cM group.

I went back and checked all of the 10 cM and over non-matches to verify that I had not made an error.  If I made errors, they were likely counting too many as NON-matches, and not the reverse, meaning I failed to visually identify matches.  However, with almost 6000 spreadsheet rows for the child, a few errors wouldn’t affect the totals significantly or even noticeably.

I hope that other people in non-endogamous populations will do the same type of double parent phasing and report on their results in the same type of format.  This experiment took about 2 days.

Furthermore, I would love to see this same type of experiment for endogamous families as well.

Summary

If you can phase your matches to either or both of your parents, absolutely, do.  This this exercise shows why, if you have only one parent to match against, you can’t just assume that anyone who doesn’t match you on your one parent’s side automatically matches you from the other parent. At least, not below about 15 cM.

Whether you can phase against your parent or not, this exercise should help you analyze your segment matches with an eye towards determining whether or not they are valid, and what different kinds of matches mean to your genealogy.

If nothing else, at least we can quantify the relatively likelihood, based on the size of the matching segment, in a non-endogamous population, a match would match a parent, if we had one to match against, meaning that they are a legitimate match.  Did you get all that?

In a nutshell, we can look at the Parent/Child Phased Match Chart produced by this exercise and say that our 8.5 cM match has about a 66% chance of being a legitimate match, and our 10.5 cM match has a 95% change of being a legitimate match.

You’re welcome.

Enjoy!!

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Concepts – Identical by…Descent, State, Population and Chance

In genetic genealogy, what does it mean when someone says they are “identical by” something…and what are those various somethings?

In autosomal DNA, where your DNA on chromosomes 1-22 (and sometimes X) is compared to other people for matches of a size that indicates a genealogical relationship, you can actually match people in different ways, for different reasons.

But first, let’s make one thing perfectly clear. There is only one way to obtain your autosomal DNA – and that’s through your parents, 50% from each parent.  However, how much of their (and your) ancestor’s DNA you receive is not necessarily half of what they received from that ancestor.

If you receive ANY DNA from that ancestor, it MUST BE through your parents. There is no other way to inherit DNA.

Period.

No. Other. Way.

If you would like to read the Concepts article about inheritance and matching, click here. If you don’t understand autosomal DNA inheritance and matching concepts, you won’t be able to understand the rest of this article.

Identical by Descent (IBD)

When you match someone because you share DNA from a common ancestor, that is called Identical by Descent, or IBD. That’s what you want.  That’s a good thing, genealogically speaking.

Let’s take a look at how an IBD segment of DNA works. In the graphic below, the strand location is in the first column.  The next two pink columns are the two strands that your mother carries, one from her Mom and one from her Dad – and the values in each location from each parent.  Columns 4 and 5 are the two blue strands of DNA carried by your Dad, one from his Mom and one from his Dad.  The final two columns are what you inherited from both your mother and your father.  In this case, we made it easy and you simply inherited one of each of their strands entirely.  Yes, that does happen in some cases for a particular chromosome segment, but not all of the time.  Conceptually, for this example, it doesn’t matter.

Identical 1

Your Inheritance

In this example, you inherited strand 1 from your Mom, all As and strand 2 from Dad, all Gs. Your match, shown in the graphic below, matches you on all As, so also matches your mother.  This phenomenon is called parental phasing, which means we know it’s a legitimate match because the person matches both you and one of your parents.

For purposes of this conceptual discussion you must match on all 10 locations for this to be considered a matching segment. So in this case, your matching threshold is “10 locations.”

Identical 2

Your Match Matches You and Your Mother’s DNA – Identical by Descent

Now, understand that while I’ve shown “You” with your strands color coded so you can see who you received which pieces of DNA from – that’s not how your DNA really looks. There is no color coding in nature.  I’ve added color coding to make understanding these concepts easier.

This is how you and your parents DNA really look:

Identical 3

Notice that in your parents, their parent’s strands are mixed back and forth, so you really can’t tell which DNA came from whom.  It’s the same for you too.

What the matching software has to do is to look for a common letter between you and your match.

So, at location 1, you inherited an A and a G from your parents. Your match has an A and a T, so you and your match share a common A.  If you look at all of your matches locations, they share a common A with you on all of those locations.  It just so happens you received that A from your mother – but without your Mom to compare to – you have no way to know which parent that particular DNA value came from.  So, the best matching software can do is to tell you that indeed, you do match – on 10 locations in a row – so this is considered a match and will be reported as such on your match list.

Why you match is another matter altogether.

And, ahem….there is another way to match someone, aside from receiving ancestral DNA from your parents. I know, this is a bad joke isn’t it.  Yes, it is, but it’s real.

So, to summarize, there is no other way to obtain your DNA except 50% from one parent and 50% from the other.

However there are two ways to match someone:

  • Identical by Descent, IBD, meaning you match someone because you share the same DNA segment that you received from an ancestor through a parent, as shown above.
  • Identical by Chance, IBC, meaning that you match someone, but randomly – not by inheritance.  How the heck can that happen?

Let’s look at how that can happen.

Identical by Chance (IBC)

Because you receive a strand of DNA from each of your parents, but that DNA is all intermixed in you, you can possibly match someone else by virtue of the fact that they aren’t actually matching your ancestral DNA segment inherited from an ancestor, but by chance they are matching DNA that bounces back and forth between your parents’ DNA.

Identical 4

Your Match Matches Neither of your Parents’ Strands of DNA – Identical by Chance

In this example, you can see the that you inherited the same strands from your parents as in example 1 above, but your match is now matching you, not on your mother’s strand 1, all As, but on a combination of A from your mother and G from your father. Therefore, they don’t match either of your parents on this segment, because they are matching you by chance and not because you share a strand of DNA that you received from a common ancestor on this segment with your match.

This is easy to discern because while they match you, they won’t match either of your parents on that segment, because the match is not on an ancestral DNA segment, passed down from an ancestor. Using parental phasing, you compare your matches to your parents to see which “side” they fall on.  If they fall on neither parents’ side, then they are IBC or identical by chance.

Identical 5

Identical By Chance Identified Through Parental Phasing

In this example, you can see that you match all of these people. By using parental phasing, you can tell that you are identical by descent (IBD) to everyone except John, who matches neither of your parents, so your match to John is identical by chance (IBC).  We will talk more in an upcoming article about Parental Phasing.

If you don’t have your parents to compare to, and you match multiple people on the same segment, there should be 2 groups of people who all match each other on that segment – one group from your Mom’s side and one from your Dad’s side – even if you can’t identify your common ancestor. If there are people who don’t fit into either of those two groups, because they don’t match those group members, then the misfits are identical by chance.

Even if your parents are unavailable, this is a situation where testing other relatives helps, and the closer the better, because those relatives will also fall into those match groups and will help identify which group is from which side of your family, and which ancestral line.

In the example below, using the same people from the phased parent example above, we no longer have our parents to compare to, but we do have an aunt, Mom’s sister, and an uncle, Dad’s brother. By comparing those who match us to our close relatives – if everyone in the match group matches each other, then we know they are IBD and the come from Mom’s side of the family or Dad’s side of the family.

Identical 6

Identical By Chance Identified Through Close Family Match Groups

In general matching, meaning not on specific segments, just on your match list, if John and I match, but John doesn’t match mother’s sister, it could mean that John matches me on a different segment that my aunt didn’t inherit from my grandparents but that my mother did. So the match could be valid, even though he doesn’t match my aunt.

However, moving to the segment matching level, shown above, we can differentiate, at least for that segment.  This is yet another example of why segment analysis tools are so critically important.

If we only had one matching group, the green above, we would not be able to say that John was IBC on this segment, because John might be matching me on Dad’s side.

But in this case, we have proof points on both sides of this same segment, with two match groups, green from Mom and blue from Dad.  Mom’s side has a match group of 4+me (including her sister) who all match each other on this same segment, indicating that they all descend through my mother’s side of my tree.  On Dad’s side, we have his brother and two other people who match each other and me on those same segments.

Since John matches no one in either match group on either side, his match to me on this segment must be IBC.  You can read more about match groups and confidence here.

Identical by chance segments tend to be smaller segments, because the chances of matching more locations in a row by chance diminish as the number of locations increases.

Ok, so now you’ve got this – the two ways to match. Identical by descent (IBD) and identical by chance (IBC,) nature’s cruel joke.

So, what the heck are identical by state (IBS) and identical by population (IBP).

Good questions.

Identical by State (IBS)

Identical by state is really an archaic term now, but you’ll likely still run into it from time to time. Understand that genetic genealogy is still a really new field of discovery.  Initially, terms weren’t defined very well and have since evolved.  IBD was used to mean a match where you could find a common ancestral line.  IBS, or identical by state, was often used when one could not find the ancestral line.  What this implied was that the match was not genealogical in nature.  But that often wasn’t true.  Just because we can’t determine who the common ancestor is, doesn’t mean that common ancestor doesn’t exist.  After we have more matches, we may well figure out the common ancestor at a later time.

What are some reasons we might not be able to figure out who our common ancestor is?

  • There’s a NPE or undocumented adoption in one line or the other.
  • The pedigree chart of one or both people doesn’t go back far enough in time.
  • The pedigree chart of one or both people is incorrect.
  • Not enough people have tested to connect the dots between the DNA. For example, we may share a common surname, Dodson, but be unable to actually pinpoint which Dodson line/ancestor we share.
  • The match is identical by population (IBP) and not in a genealogical timeframe. We see this most often in highly endogamous populations.
  • The match is identical by chance (IBC) and there is no common ancestor.

The tendency in the past has been to assume that if you can’t find the ancestor, then the problem MUST be that the match is Identical by State. But the problem is that identical by state includes two categories that are mutually exclusive; Identical by Chance and Identical by Population.

Identical by chance means there is no common ancestor, as we illustrated above.

Identical by Population means there IS a common ancestor, and you did receive your DNA from that ancestor, but you may not be able to figure out who it was because it’s too far back in time and many people from that same population base share that DNA segment.

So, today, we don’t say IBS anymore, we say either IBD and if it’s not IBD then it’s either IBC or IBP, but not IBS. If someone says IBS, you need to ask and see if you can determine whether they mean, IBC or IBP, or if they are trying to say something else like “I can’t identify the common ancestor so it must be IBS.”

Identical by Population (IBP)

Identical by population means that a large portion of a population group shares a particular segment of DNA. Some people feel IBP segments are not useful and want all of these segments to be stripped away by population (or academic) based phasing software.

In some cases, if an individual is 100% Jewish, for example, they will have many IBP segments from within the highly endogamous Jewish population. They don’t have any other ancestral DNA segments from ancestors who aren’t Jewish to contrast against in their DNA, so their IBP segments are not useful to them, and are in fact, just in the opposite.  There are too many IBP segments and they are in the way – often referred to as “noise” because they are not genealogically useful, even though they are descended from an ancestor (IBD).  So, yes, IBP is a subset of IBD.

However, for someone who has the following genealogy, these same population based endogamous segments can be extremely useful and informative.

Identical 7

In this conceptual pedigree chart, the Jewish person married a non-Jewish person with deep colonial American ancestry. Their child “Colonial Jew” married someone who was mixed “Irish Asian.”  The person at the bottom, “me,” is not themselves endogamous but has several widely variant lines in their heritage including endogamous lines.

If I’m lucky enough to have an African population segment, that tells me very clearly which genealogical line that match is probably from. But if those IBP segments are removed, they can’t inform me in this situation.

Same with Jewish, or Asian, or Native American.

Let’s see how this might work in real matching.

Let’s say your mother’s A value is only found in African populations, and it’s found in very high proportions in African populations and much less frequently anyplace else in the world, except for where Africans settled.

Identical 8

Identical By Population Example Where Mother’s A Equals African

A few match outcomes are possible:

  1. You match with someone and you can discern a common ancestor or at least an ancestral line because you have only one African genealogical line – an ancestor in your mother’s line, like in the pedigree chart above.
  2. You match with someone and you cannot discern a common ancestor because many or all of your lines are African, similar to the Jewish example.
  3. You match with someone and you identify a common ancestor, but later a second genealogical line matches on that same segment because the segment is so common in the African population. This means you could have received that actual DNA segment from either ancestral line.
  4. Some DNA testing company runs academic or population based phasing software against your DNA and removes that segment entirely because they’ve decided that it occurs too frequently in a population to be useful. In this case, you won’t match that person at all.
  5. Some DNA testing company runs academic or population based phasing software against your DNA and removes that segment entirely because they’ve decided that particular segment in your results is “too matchy” so it must therefore be “invalid” and population based. This is often referred to as a “pile-up” and means that you have proportionally more matches on that segment than you do on other segments. If your “pile-up” segments are removed in this case, again, you won’t match at all. This is exactly what happened to my Acadian matches when Ancestry implemented their Timber phasing software, which removes pile-ups.

The graph below was provided to me at Ancestry DNA Day as an example of my own “pile-up” areas in my genome.

genome pileups

Ancestry with their Timber routine uses population phasing and removes your areas they deem “too matchy”? This helps Jewish and other heavily endogamous people by removing truly population based matches that are spurious and the contributing ancestor impossible to discern.  An endogamous individual could achieve much of the same effect by utilizing a higher matching threshold for their own matches, although that’s not an option at Ancestry.

However, for those of us who are not entirely endogamous, but who may have endogamous lines or lines from different parts of the world, population based phasing removes valuable informational segments and therefore, prevents valuable matches. When Ancestry ran Timber against my results, I lost all but one of my Acadian matches.  Yes, Acadians are heavily endogamous, but in my case, that line accounts for 1 of my 16 great-great-grandparents.  Believe me, if I had a tool to put all of my autosomal matches in one of 16 buckets, I would think it was a wonderful day!!!

16 gggrandparents

Because of endogamy, I actually carried MORE Acadian DNA that I would otherwise carry from a non-endogamous population – so yes, I am very matchy to my Acadian cousins, especially on smaller segments – or I was until Ancestry stripped all of that way.  Thankfully, I still have all of my matches at Family Tree DNA.

Why is endogamous DNA more matchy? Because endogamous populations only have the founders’ DNA and they just keep passing the same founder DNA around and around.

Ironically, another word for this kind of phasing is called “excess IBD” phasing. This means that “someone” decides unilaterally how much matching one “should” have and just chops the rest off at that threshold.  Clearly, that threshold for a fully Jewish person and me would be very different – and one size absolutely does NOT fit all.

I want to show you one more example of what population based phasing does. It chops the heart out of segments that would otherwise match.

People whose parents also test should match their parents on exactly 22 segments, one for each chromosome – because each child is a 100% match to their parents. If there is a read error or two (or three), then let’s say they could have as many as 25 matches, because some chromosomes are chopped in two because of a technical issue.  It occasionally happens.

At Ancestry, we’re seeing 80 to 120 matches for each parent/child pair, which means Timber is removing 58 to roughly 100 legitimate segments that you received from your parent.  One individual reported that they match one parent on 150 different segments, meaning that Ancestry removed 128 segments they decided are “too matchy” but are very clearly ancestral, or IBD, because all of your DNA must match your parents DNA on the strand they gave you.  However because of Timber’s removal of “too matchy” segments, the person no longer matches their parent on that removed segment – or on any of those 58 to 128 removed segments.  And remember, there is only one way to receive your DNA, so all of your DNA must match that of your parents.  You have no invalid matches to your parents DNA.  You can read more here.

Here’s a visual of what IBP phased matching does to you. Recall in our example that you need 10 contiguous matching locations to be considered a match.  I’m showing 20 locations in this example.

Identical 9

Normal Matching – No Population or Academic Phasing

In this first example, the DNA you inherited from your mother is a combination of T and A, where A=African. Notice that only part of what you inherited from your mother is the A this time.

In normal matching without IBP phasing, above, the matching threshold is still 10, but you match your match on a segment that totals 20 locations or units. Now it’s up to you to see if you can identify your common ancestor.

In the IBP phased example, below, your African DNA is removed as a result of population based phasing software. Your African DNA used to be where the red spot with no values is showing in the You 1 column.  Therefore, you still match on the Ts, but you only have a contiguous run of 7 Ts, then the 7 As phasing deleted, then 6 more matching Ts.  The problem is, of course, that instead of a nice matching segment of 20 units, above, you now have no match at all because you don’t have 10 matching locations in a row.  Of course, the same IBP phasing would apply to your mother, so your match would not match your mother either, which means that a valid parentally phased match is not reported.

Identical 10

Population Based Phased Matching Example Removing African

What’s worse, you’ll never have that opportunity to see if you can find your common ancestor, because you and your match will never be reported as a match. This is a lost opportunity.  In the first “normal matching” example, you may never BE able to find that common ancestor, but you have the opportunity to try.  In the second IBP phased matching example, you certainly won’t ever find your common ancestor because you’re not shown as a match.  When population based or academic phasing is involved, you’ll never know what you are missing.

This chopping phenomenon is not a rare occurrence with population based phasing. In fact, if you divide 100 removed segments by 22 chromosomes, there are approximately 4 artificial “chops” taken out of every one of your 22 chromosomes with each parent at Ancestry, and in some cases, more.  The person who now matches their parent on 150 segments has an average of 5.8 artifical phasing induced chops in each chromosome.  When Ancestry implemented Timber, many people lost between 80% and 90% of their total matches.  Mine went from 13,100 to 3,350, a loss of about 75%.  At least some of those were valid and we had identified common ancestral lines.

So, identical by population (IBP) doesn’t necessarily mean bad, unless you’re entirely endogamous. If you’re entirely endogamous, then IBP means challenging and can generally be overcome by looking at larger matching segments, which are less likely to be either IBP or IBC.

Identical by population can be very useful in someone not entirely endogamous in that it preserves ancestral DNA in a given population. In people who carry a combination of different endogamous lines, such as Jewish and Acadian, this phenomenon can actually be very useful, because it increases your chances of matching other individuals from that ancestral line – and being able to assign them appropriately.

Identical by What?

So, in summary, you are either identical because you received DNA from a common ancestor (IBD) or identical by chance (IBC) because nature is playing a mean joke on you and you match, literally, by chance because your match’s DNA is zigzagging back and forth between your parents’ DNA.  And by the way, you can match someone IBD on one segment and the same person IBC or IBP on others.

If you match someone but that person does not also match either of your parents, then it’s an IBC, identical by chance, match. Measuring a match against both yourself and your parents to determine if the match is IBC or IBD is called parental phasing.  We will have a Concepts article shortly about Parental Phasing, so stay tuned.

If you don’t have parents to match against, your matches on any segment should cleanly cluster into two matching groups where you match them and your matches also match each other on that same segment. One group for your mother’s side and one group for your father’s side.  Those who match you but don’t fall into one group or the other are identical by chance, like John in our example.  Of course, you won’t be able to sort these out until you have several matches on that segment.  This is also why testing all available upstream family members is so useful.

If you’re not IBC, you’re IBD meaning that you and your match received that DNA segment from a common ancestor, whether or not you can identify that ancestor.

Identical by population (IBP) is a type or subset of identical by descent (IBD) where many people from that same population group carry the same DNA segment. This is seen in its most pronounced fashion in heavily endogamous populations such as Ashkenazi Jews.

If you are from a highly endogamous population, you will have many IBP matches, generally on smaller segments that have been chopped up over time, and you will want to use a higher matching threshold, perhaps up to 10cM, for genealogical matching, or higher.

If you have endogamous lines in your tree, but are not entirely endogamous, IBP segments may actually be beneficial because you may be able to attribute matches to a specific line, even if not the specific ancestor in that line.

The smaller the segment, the more likely it is to be less useful to you, whether IBD or IBP – but that isn’t to say all small segments should be disregarded because they are assumed to be either IBC or not useful. That’s not the case.  Some are IBD and all IBD segments have the potential to be very useful.  Kitty Cooper just recently reported another wonderful success story using a 6cM triangulated segment.

If you’re highly endogamous, or only looking only for the low hanging fruit, which is more likely to be immediately rewarding, then work with only larger segment matches. They are less likely to be IBC or IBP and more likely to yield results more quickly.  I always begin with the largest matching segments, because not only are they easier to assign to an ancestor, but those matching people may also have smaller matching segments that I can tentatively (pending triangulation) attribute to that specific ancestor as well.

Here’s a handy-dandy cheat sheet if you’re having trouble remembering “Identical by What.”

Identical by Chart

Understand that working with genetic genealogy and autosomal DNA is much like panning for gold. You may get lucky and find a large nugget or two smiling at you from on top the pile, but the majority of your rewards will be as a result of hard work sifting and panning and accumulating those small golden flakes that aren’t immediately obvious and useful.  Cumulatively, they may well hold your family secrets and the keys to locks long ago frozen shut.

Here’s hoping all your matches are IBD!!!!!

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Disclosure

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