Duplicate Copies of Parental Chromosomes – Uniparental Disomy

Recently, three articles were been published that discuss a phenomenon where unsuspecting individuals have two copies one parent’s chromosome, and no copy of the other parent’s chromosome. This is called Uniparental Disomy.

Since then, online I’ve seen this phenomenon being offered as a reason for all kinds of things – which just isn’t the case.

I’m sure in part it’s because people either haven’t actually read the articles, or they don’t understand what’s being said.

I’m going to explain this briefly and then tell you how you can find out if this situation actually DOES apply to you.

Uniparental Disomy in Brief

Here are a few summary bullet points about uniparental disomy:

  • Uniparental disomy is found on ONLY ONE CHROMOSOME in roughly 1 in 2000 people in the reference samples utilized at 23andMe.
  • This is not a new discovery, per se. It was known and previously believed to occur in 1 of 3,500 births, but that frequency has been updated to 1 in 2,000 in the paper.
  • Uniparental disomy was found in 1 of 50,000 people on TWO CHROMOSOMES.
  • This is NOT the reason you have more maternal or paternal matches, in general. Legitimate reasons for more matches on one parent’s line include the fact that one family or another historically has more or fewer descendants, more or fewer dead ends, recent immigrants, ancestors from regions where DNA testing is not popular and/or endogamous populations.
  • The people included in the research were trios where the tester and their parents have all 3 tested.
  • Many/most people with uniparental disomy have no known health issues.
  • The testers have in some cases been associated with some conditions, as described in the paper and supplemental information.
  • Of the people who carry this condition, more people carry a double maternal chromosome than a double paternal chromosome.
  • Uniparental disomy occurs more on chromosome 16 than any other chromosome, twice as often as the second highest, chromosome 7, with 40 and 20 occurrences each, respectively. Chromosome 18 had none. No, no one knows why.
  • It’s not necessary for the entire chromosome to be duplicated. In some cases, only part of the chromosome is improperly combined.

Articles

This Atlantic article provides an overview:

This academic paper in Cell is referenced in The Atlantic article and is where the meat of the information is found. Be sure to look at the supplemental files too.

Much of the data for the article was from 23andMe who discussed this study in their blog here.

What About You?

Do you have a chromosome that has experienced uniparental disomy? Probably not, but there’s a very easy way for you to find out.

If you have a duplicate chromosome, or portion of a chromosome from one parent, the genetic genealogy “indicator” that you’ll see is called ROH, or Run of Homozygosity. This condition occurs in situations where you have a duplicate chromosome, or where your parents are related to each other

  1. The first question to ask yourself is whether or not your parents are related to each other. If so, you will have some ROH segments.
  2. The second question is whether you have an entire duplicated chromosome when your parents aren’t related.

In order to answer both questions, we use the tool at GedMatch called “Are your parents related?”

Are Your Parents Related to Each Other?

You’ll need to establish an account at GedMatch and upload your DNA results from one of the testing vendors.

Here are instructions for how to download from the various vendors:

Using the “Are your parents related” Tool

To use this tool at GedMatch, after your uploaded kit is finished processing, click on “Are your parents related?” and enter the kit number of the person you want to evaluate. I’m assuming for this discussion that person is you.

Parents related.png

Normally, we use this tool to determine if someone’s parents are related to each other. We find this occurring in endogamous populations or where cousins married in the past few generations, as happened rather routinely in history.

In those situations, across all of a person’s chromosomes (not just one), we find relatively small segments of common DNA inherited by the person on both their maternal and paternal copies of each chromosome.

Parents are related.png

These matching areas are called ROH or “runs of homozygosity” meaning that the DNA is identical on both chromosomes for short segments, as shown above in the regions where the top bars are solid green and the bottom bar is solid blue.

The legend for reading the graphic is shown below.

Parents related legend.png

The chromosomes of a person whose parents are not related is shown below. Notice that there are no significant green bars on top, and no blue bars on the bottom.

Parents not related.png

Simple chance alone is responsible for tiny segments that are identical, like those tiny green slivers, but not larger segments over 7cM as shown in the first example and marked by blue on the bottom.

For someone that has a fully duplicated chromosome, meaning uniparental disomy, we see something different.

A Duplicate Chromosome

For someone that has a duplicate parental chromosome, all of their chromosomes look normal except that one entire chromosome, or a very large segment, is entirely identical.

Below is an example of a person whose chromosome 7 is duplicated. The rest of this person’s chromosomes looked like the image above with only tiny green slivers.

Parents uniparental disomy.png

If you have a duplicate chromosome, you’re rare, one in every 2,000 people in the populations studied.

If you have two identical chromosomes, you’re hen’s teeth rare – 1 in 50,000.

If you have uniparental disomy, you probably have no idea. You can also experience uniparental disomy when most of, but not all of a single chromosome is duplicated.

If you have duplicate parental chromosomes, you’ll match people on both sides of your family normally on all of your OTHER non-duplicate chromosomes. On your duplicate chromosome, you’ll only match people from the parent whose chromosome is duplicated.

In other words, this is NOT why you seem to be missing matches from one side of your family generally. You’ll need to look at other reasons to explain that.

If you have a duplicate chromosome, or large segment of a duplicate chromosome, leave a comment.

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First Steps When Your DNA Results are Ready – Sticking Your Toe in the Genealogy Water

First steps helix

Recently someone asked me what the first steps would be for a person who wasn’t terribly familiar with genealogy and had just received their DNA test results.

I wrote an article called DNA Results – First Glances at Ethnicity and Matching which was meant to show new folks what the various vendor interfaces look like. I was hoping this might whet their appetites for more, meaning that the tester might, just might, stick their toe into the genealogy waters😊

I’m hoping this article will help them get hooked! Maybe that’s you!

A Guide

This article can be read in one of two ways – as an overview, or, if you click the links, as a pretty thorough lesson. If you’re new, I strongly suggest reading it as an overview first, then a second time as a deeper dive. Use it as a guide to navigate your results as you get your feet wet.

I’ll be hotlinking to various articles I’ve written on lots of topics, so please take a look at details (eventually) by clicking on those links!

This article is meant as a guideline for what to do, and how to get started with your DNA matching results!

If you’re looking for ethnicity information, check out the First Glances article, plus here and here and here.

Concepts – Calculating Ethnicity Percentages provides you with guidelines for how to estimate your own ethnicity percentages based on your known genealogy and Ethnicity Testing – A Conundrum explains how ethnicity testing is done.

OK, let’s get started. Fun awaits!

The Goal

The goal for using DNA matching in genealogy depends on your interests.

  1. To discover cousins and family members that you don’t know. Some people are interested in finding and meeting relatives who might have known their grandparents or great-grandparents in the hope of discovering new family information or photos they didn’t know existed previously. I’ve been gifted with my great-grandparent’s pictures, so this strategy definitely works!
  2. To confirm ancestors. This approach presumes that you’ve done at least a little genealogy, enough to construct at least a rudimentary tree. Ancestors are “confirmed” when you DNA match multiple other people who descend from the same ancestor through multiple children. I wrote an article, Ancestors: What Constitutes Proof?, discussing how much evidence is enough to actually confirm an ancestor. Confirmation is based on a combination of both genealogical records and DNA matching and it varies depending on the circumstances.
  3. Adoptees and people with unknown parents seeking to discover the identities of those people aren’t initially looking at their own family tree – because they don’t have one yet. The genealogy of others can help them figure out the identity of those mystery people. I wrote about that technique in the article, Identifying Unknown Parents and Individuals Using DNA Matching.

DNAAdoption for Everyone

Educational resources for adoptees and non-adoptees alike can be found at www.dnaadoption.org. DNAAdoption is not just for adoptees and provides first rate education for everyone. They also provide trained and mentored search angels for adoptees who understand the search process along with the intricacies of navigating the emotional minefield of adoption and unknown parent searches.

First Look” classes for each vendor are free for everyone at DNAAdoption and are self-paced, downloadable onto your computer as a pdf file. Intro to DNA, Applied Autosomal DNA and Y DNA Basics classes are nominally priced at between $29 and $49 and I strongly recommend these. DNAAdoption is entirely non-profit, so your class fee or contribution supports their work. Additional resources can be found here and their 12 adoptee search steps here.

Ok, now let’s look at your results.

Matches are the Key

Regardless of your goal, your DNA matches are the key to finding answers, whether you want to make contact with close relatives, prove your more distant ancestors or you’re involved in an adoptee or unknown parent search.

Your DNA matches that of other people because each of you inherited a piece of DNA, called a segment, where many locations are identical. The length of that DNA segment is measured in centiMorgans and those locations are called SNPs, or single nucleotide polymorphisms. You can read about the definition of a centimorgan and how they are used in the article Concepts – CentiMorgans, SNPs and Pickin’Crab.

While the scientific details are great, they aren’t important initially. What is important is to understand that the more closely you match someone, the more closely you are related to them. You share more DNA with close relatives than more distant relatives.

For example, I share exactly half of my mother’s DNA, but only about 25% of each of my grandparents’ DNA. As the relationships move further back in time, I share less and less DNA with other people who descend from those same ancestors.

Informational Tools

Every vendor’s match page looks different, as was illustrated in the First Glances article, but regardless, you are looking for four basic pieces of information:

  • Who you match
  • How much DNA you share with your match
  • Who else you and your match share that DNA with, which suggests that you all share a common ancestor
  • Family trees to reveal the common ancestor between people who match each other

Every vendor has different ways of displaying this information, and not all vendors provide everything. For example, 23andMe does not support trees, although they allow you to link to one elsewhere. Ancestry does not provide a tool called a chromosome browser which allows you to see if you and others match on the same segment of DNA. Ancestry only tells you THAT you match, not HOW you match.

Each vendor has their strengths and shortcomings. As genealogists, we simply need to understand how to utilize the information available.

I’ll be using examples from all 4 major vendors:

Your matches are the most important information and everything else is based on those matches.

Family Tree DNA

I have tested many family members from both sides of my family at Family Tree DNA using the Family Finder autosomal test which makes my matches there incredibly useful because I can see which family members, in addition to me, my matches match.

Family Tree DNA assigns matches to maternal and paternal sides in a unique way, even if your parents haven’t tested, so long as some close relatives have tested. Let’s take a look.

First Steps Family Tree DNA matches.png

Sign on to your account and click to see your matches.

At the top of your Family Finder matches page, you’ll see three groups of things, shown below.

First Steps Family Tree DNA bucketing

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A row of tools at the top titled Chromosome Browser, In Common With and Not in Common With.

A second row of tabs that include All, Paternal, Maternal and Both. These are the maternal and paternal tabs I mentioned, meaning that I have a total of 4645 matches, 988 of which are from my paternal side and 847 of which are from my maternal side.

Family Tree DNA assigns people to these “buckets” based on matches with third cousins or closer if you have them attached in your tree. This is why it’s critical to have a tree and test close relatives, especially people from earlier generations like aunts, uncles, great-aunts/uncles and their children if they are no longer living.

If you have one or both parents that can test, that’s a wonderful boon because anyone who matches you and one of your parents is automatically bucketed, or phased (scientific term) to that parent’s side of the tree. However, at Family Tree DNA, it’s not required to have a parent test to have some matches assigned to maternal or paternal sides. You just need to test third cousins or closer and attach them to the proper place in your tree.

How does bucketing work?

Maternal or Paternal “Side” Assignment, aka Bucketing

If I match a maternal first cousin, Cheryl, for example, and we both match John Doe on the same segment, John Doe is automatically assigned to my maternal bucket with a little maternal icon placed beside the match.

First Steps Family Tree DNA match info

Click to enlarge

Every vendor provides an estimated or predicted relationship based on a combination of total centiMorgans and the longest contiguous matching segment. The actual “linked relationship” is calculated based on where this person resides in your tree.

The common surnames at far right are a very nice features, but not every tester provides that information. When the testers do include surnames at Family Tree DNA, common surnames are bolded. Other vendors have similar features.

People with trees are shown near their profile picture with a blue pedigree icon. Clicking on the pedigree icon will show you their ancestors. Your matches estimated relationship to you indicates how far back you should expect to share an ancestor.

For example, first cousins share grandparents. Second cousins share great-grandparents. In general, the further back in time your common ancestor, the less DNA you can be expected to share.

You can view relationship information in chart form in my article here or utilize DNAPainter tools, here, to see the various possibilities for the different match levels.

Clicking on the pedigree chart of your match will show you their tree. In my tree, I’ve connected my parents in their proper places, along with Cheryl and Don, mother’s first cousins. (Yes, they’ve given permission for me to utilize their results, so they aren’t always blurred in images.)

Cheryl and Don are my first cousins once removed, meaning my mother is their first cousin and I’m one generation further down the tree. I’m showing the amount of DNA that I share with each of them in red in the format of total DNA shared and longest unbroken segment, taken from the match list. So 382-53 means I share a total of 382 cM and 53 cM is the longest matching block.

First Steps Family Tree DNA tree.png

The Chromosome Browser

Utilizing the chromosome browser, I can see exactly where I match both Don and Cheryl. It’s obvious that I match them on at least some different pieces of my DNA, because the total and longest segment amounts are different.

The reason it’s important to test lots of close relatives is because even siblings inherit different pieces of DNA from their parents, and they don’t pass the same DNA to their offspring either – so in each generation the amount of shared DNA is probably reduced. I say probably because sometimes segments are passed entirely and sometimes not at all, which is how we “lose” our ancestors’ DNA over the generations.

Here’s a matching example utilizing a chromosome browser.

First Steps Family Tree DNA chromosome browser.png

I clicked the checkboxes to the left of both Cheryl and Don on the match page, then the Chromosome Browser button, and now you can see, above, on chromosomes 1-16 where I match Cheryl (blue) and Don (red.)

In this view, both Don and Cheryl are being compared to me, since I’m the one signed in to my account and viewing my DNA matches. Therefore, one of the bars at each chromosome represents Don’s DNA match to me and one represents Cheryl’s. Cheryl is the first person and Don is the second. Person match colors (red and blue) are assigned arbitrarily by the system.

My grandfather and Cheryl/Don’s father, Roscoe, were siblings.

You can see that on some segments, my grandfather and Roscoe inherited the same segment of DNA from their parents, because today, my mother gave me that exact same segment that I share with both Don and Cheryl. Those segments are exactly identical and shown in the black boxes.

The only way for us to share this DNA today is for us to have shared a common ancestor who gave it to two of their children who passed it on to their descendants who DNA tested today.

On other segments, in red boxes, I share part of the same segments of DNA with Cheryl and Don, but someone along the line didn’t inherit all of that segment. For example on chromosome 3, in the red box, you can see that I share more with Cheryl (blue) than Don (red.)

In other cases, I share with either Don or Cheryl, but Don and Cheryl didn’t inherit that same segment of DNA from their father, so I don’t share with both of them. Those are the areas where you see only blue or only red.

On chromosome 12, you can see where it looks like Don’s and Cheryl’s segments butt up against each other. The DNA was clearly divided there. Don received one piece and Cheryl got the other. That’s known as a crossover and you can read about crossovers here, if you’d like.

It’s important to be able to view segment information to be able to see how others match in order to identify which common ancestor that DNA came from.

In Common With

You can use the “In Common With” tool to see who you match in common with any match. My first 6 matches in common with Cheryl are shown below. Note that they are already all bucketed to my maternal side.

First Steps Family Tree DNA in common with

click to enlarge

You can click on up to 7 individuals in the check box at left to show them on the chromosome browser at once to see if they match you on common segments.

Each matching segment has its own history and may descend from a different ancestor in your common tree.

First Steps 7 match chromosome browser

click to enlarge

If combinations of people do match me on a common segment, because these matches are all on my maternal side, they are triangulated and we know they have to descend from a common ancestor, assuming the segment is large enough. You can read about the concept of triangulation here. Triangulation occurs when 3 or more people (who aren’t extremely closely related like parents or siblings) all match each other on the same reasonably sized segment of DNA.

If you want to download your matches and work through this process in a spreadsheet, that’s an option too.

Size Matters

Small segments can be identical by chance instead of identical by descent.

  • “Identical by chance” means that you accidentally match someone because your DNA on that segment has been combined from both parents and causes it to match another person, making the segment “looks like” it comes from a common ancestor, when it really doesn’t. When DNA is sequenced, both your mother and father’s strands are sequenced, meaning that there’s no way to determine which came from whom. Think of a street with Mom’s side and Dad’s side with identical addresses on the houses on both sides. I wrote about that here.
  • “Identical by descent” means that the DNA is identical because it actually descends from a common ancestor. I discussed that concept in the article, We Match, But Are We Related.

Generally, we only utilize 7cM (centiMorgan) segments and above because at that level, about half of the segments are identical by descent and about half are identical by chance, known as false positives. By the time we move above 15 cM, most, but not all, matches are legitimate. You can read about segment size and accuracy here.

Using “In Common With” and the Matrix

“In Common With” is about who shares DNA. You can select someone you match to see who else you BOTH match. Just because you match two other people doesn’t necessarily mean that it’s on the same segment of DNA. In fact, you could match one person from your mother’s side and the other person from your father’s side.

First Steps match matrix.png

In this example, you match Person B due to ancestor John Doe and Person C due to ancestor Susie Smith. However, Person B also matches person C, but due to ancestor William West that they share and you don’t.

This example shows you THAT they match, but not HOW they match.

The only way to assure that the matches between the three people above are due to the same ancestor is to look at the segments with a chromosome browser and compare all 3 people to each other. Finding 3 people who match on the same segment, from the same side of your tree means that (assuming a reasonably large segment) you share a common ancestor.

Family Tree DNA has a nice matrix function that allows you to see which of your matches also match each other.

First steps matrix link

click to enlarge

The important distinction between the matrix and the chromosome browser is that the chromosome browser shows you where your matches match you, but those matches could be from both sides of your tree, unless they are bucketed. The matrix shows you if your matches also match each other, which is a huge clue that they are probably from the same side of your tree.

First Steps Family Tree DNA matrix.png

A matrix match is a significant clue in terms of who descends from which ancestors. For example, I know, based on who Amy matches, and who she doesn’t match, that she descends from the Ferverda side and that Charles, Rex and Maxine descend from ancestors on the Miller side.

Looking in the chromosome browser, I can tell that Cheryl, Don, Amy and I match on some common segments.

Matching multiple people on the same segment that descends from a common ancestor is called triangulation.

Let’s take a look at the MyHeritage triangulation tool.

MyHeritage

Moving now to MyHeritage who provides us with an easy to use triangulation tool, we see the following when clicking on DNA matches on the DNA tab on the toolbar.

First Steps MyHeritage matches

click to enlarge

Cousin Cheryl is at MyHeritage too. By clicking on Review DNA Match, the purple button on the right, I can see who else I match in common with Cheryl, plus triangulation.

The list of people Cheryl and I both match is shown below, along with our relationships to each person.

First Steps MyHeritage triangulation

click to enlarge

I’ve selected 2 matches to illustrate.

The first match has a little purple icon to the right which means that Amy triangulates with me and Cheryl.

The second match, Rex, means that while we both match Rex, it’s not on the same segment. I know that without looking further because there is no triangulation button. We both match Rex, but Cheryl matches Rex on a different segment than I do.

Without additional genealogy work, using DNA alone, I can’t say whether or not Cheryl, Rex and I all share a common ancestor. As it turns out, we do. Rex is a known cousin who I tested. However, in an unknown situation, I would have to view the trees of those matches to make that determination.

Triangulation

Clicking on the purple triangulation icon for Amy shows me the segments that all 3 of us, me, Amy and Cheryl share in common as compared to me.

First Steps MyHeritage triangulation chromosome browser.png

Cheryl is red and Amy is yellow. The one segment bracketed with the rounded rectangle is the segment shared by all 3 of us.

Do we have a common ancestor? I know Cheryl and I do, but maybe I don’t know who Amy is. Let’s look at Amy’s tree which is also shown if I scroll down.

First Steps MyHeritage common ancestor.png

Amy didn’t have her tree built out far enough to show our common ancestor, but I immediately recognized the surname Ferveda found in her tree a couple of generations back. Darlene was the daughter of Donald Ferverda who was the son of Hiram Ferverda, my great-grandfather.

Hiram was the father of Cheryl’s father, Roscoe and my grandfather, John Ferverda.

First Steps Hiram Ferverda pedigree.png

Amy is my first cousin twice removed and that segment of DNA that I share with her is from either Hiram Ferverda or his wife Eva Miller.

Now, based on who else Amy matches, I can probably tell whether that segment descends from Hiram or Eva.

Viva triangulation!

Theory of Family Relativity

MyHeritage’s Theory of Family Relativity provides theories to people whose DNA matches regarding their common ancestor if MyHeritage can calculate how the 2 people are potentially related.

MyHeritage uses a combination of tools to make that connection, including:

  • DNA matches
  • Your tree
  • Your match’s tree
  • Other people’s trees at MyHeritage, FamilySearch and Geni if the common ancestor cannot be found in your tree compared against your DNA match’s MyHeritage
  • Documents in the MyHeritage data collection, such as census records, for example.

MyHeritage theory update

To view the Theories, click on the purple “View Theories” banner or “View theory” under the DNA match.

First Steps MyHeritage theory of relativity

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The theory is displayed in summary format first.

MyHeritage view full theory

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You can click on the “View Full Theory” to see the detail and sources about how MyHeritage calculated various paths. I have up to 5 different theories that utilize separate resources.

MyHeritage review match

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A wonderful aspect of this feature is that MyHeritage shows you exactly the information they utilized and calculates a confidence factor as well.

All theories should be viewed as exactly that and should be evaluated critically for accuracy, taking into consideration sources and documentation.

I wrote about using Theories of Relativity, with instructions, here and here.

I love this tool and find the Theories mostly accurate.

AncestryDNA

Ancestry doesn’t offer a chromosome browser or triangulation but does offer a tree view for people that you match, so long as you have a subscription. In the past, a special “Light” subscription for DNA only was available for approximately $49 per year that provided access to the trees of your DNA matches and other DNA-related features. You could not order online and had to call support, sometimes asking for a supervisor in order to purchase that reduced-cost subscription. The “Light” subscription did not provide access to anything outside of DNA results, meaning documents, etc. I don’t know if this is still available.

After signing on, click on DNA matches on the DNA tab on the toolbar.

You’ll see the following match list.

First Steps Ancestry matches

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I’ve tested twice at Ancestry, the second time when they moved to their new chip, so I’m my own highest match. Click on any match name to view more.

First Steps Ancestry shared matches

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You’ll see information about common ancestors if you have some in your trees, plus the amount of shared DNA along with a link to Shared Matches.

I found one of the same cousins at Ancestry whose match we were viewing at MyHeritage, so let’s see what her match to me at Ancestry looks like.

Below are my shared matches with that cousin. The notes to the right are mine, not provided by Ancestry. I make extensive use of the notes fields provided by the vendors.

First Steps Ancestry shared matches with cousin

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On your match list, you can click on any match, then on Shared Matches to see who you both match in common. While Ancestry provides no chromosome browser, you can see the amount of DNA that you share and trees, if any exist.

Let’s look at a tree comparison when a common ancestor can be detected in a tree within the past 7 generations.

First Steps Ancestry view ThruLines.png

What’s missing of course is that I can’t see how we match because there’s no chromosome browser, nor can I see if my matches match each other.

Stitched Trees

What I can see, if I click on “View ThruLines” above or ThruLines on the DNA Summary page on the main DNA tab is all of the people I match who Ancestry THINKS we descend from a common ancestor. This ancestor information isn’t always taken from either person’s tree.

For example, if my match hadn’t included Hiram Ferverda in her tree, Ancestry would use other people’s trees to “stitch them together” such that the tester is shown to be descended from a common ancestor with me. Sometimes these stitched trees are accurate and sometimes they are not, although they have improved since they were first released. I wrote about ThruLines here.

First Steps Ancestry ThruLines tree

click to enlarge

In closer generations, especially if you are looking to connect with cousins, tree matching is a very valuable tool. In the graphic above, you can see all of the cousins who descend from Hiram Ferverda who have tested and DNA match to me. These DNA matches to me either descend from Hiram according to their trees, or Ancestry believes they descend from Hiram based on other people’s trees.

With more distant ancestors, other people’s trees are increasingly likely to be copied with no sources, so take them with a very large grain of salt (perchance the entire salt lick.) I use ThruLines as hints, not gospel, especially the further back in time the common ancestor. I wish they reached back another couple of generations. They are great hints and they end with the 7th generation where my brick walls tend to begin!

23andMe

I haven’t mentioned 23andMe yet in this article. Genealogists do test there, especially adoptees who need to fish in every pond.

23andMe is often the 4th choice of the major 4 vendors for genealogy due to the following challenges:

  • No tree support, other than allowing you to link to a tree at FamilySearch or elsewhere. This means no tree matching.
  • Less than 2000 matches, meaning that every person is limited to a maximum of 2000 matches, minus however many of those 2000 don’t opt-in for genealogical matching. Given that 23andMe’s focus is increasingly health, my number of matches continues to decrease and is currently just over 1500. The good news is that those 1500 are my highest, meaning closest matches. The bad news is the genealogy is not 23andMe’s focus.

If you are an adoptee, a die-hard genealogist or specifically interested in ethnicity, then test at 23andMe. Otherwise all three of the other vendors would be better choices.

However, like the other vendors, 23andMe does have some features that are unique.

Their ethnicity predictions are acknowledged to be excellent. Ethnicity at 23andMe is called Ancestry Composition, and you’ll see that immediately when you sign in to your account.

First Steps 23andMe DNA Relatives.png

Your matches at 23andMe are found under DNA Relatives.

First Steps 23andMe tools

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At left, you’ll find filters and the search box.

Mom’s and Dad’s side filter matches if you’ve tested your parents, but it’s not like the Family Tree DNA bucketing that provides maternal and paternal side bucketing by utilizing through third cousins if your parents aren’t available for testing.

Family names aren’t your family names, but the top family names that match to you. Guess what my highest name is? Smith.

However, Ancestor Birthplaces are quite useful because you can sort by country. For example, my mother’s grandfather Ferverda was born in the Netherlands.

First Steps 23andMe country.png

If I click on Netherlands, I can see my 5 matches with ancestors born in the Netherlands. Of course, this doesn’t mean that I match because of my match’s Dutch ancestors, but it does provide me with a place to look for a common ancestor and I can proceed by seeing who I match in common with those matches. Unfortunately, without trees we’re left to rely on ancestor birthplaces and family surnames, if my matches have entered that information.

One of my Dutch matches also matches my Ferverda cousin. Given that connection, and that the Ferverda family immigrated from Holland in 1868, that’s a starting point.

MyHeritage has a similar features and they are much more prevalent in Europe.

By clicking on my Ferverda cousin, I can view the DNA we share, who we match in common, our common ethnicity and more. I have the option of comparing multiple people in the chromosome browser by clicking on “View DNA Comparison” and then selecting who I wish to compare.

First Steps 23andMe view DNA Comparison.png

By scrolling down instead of clicking on View DNA Comparison, I can view where my Ferverda cousin matches me on my chromosomes, shown below.

First STeps 23andMe chromosome browser.png

23andMe identifies completely identical segments which would be painted in dark purple, the legend at bottom left.

Adoptees love this feature because it would immediately differentiate between half and full siblings. Full siblings share approximately 25% of the exact DNA on both their maternal and paternal strands of DNA, while half siblings only share the DNA from one parent – assuming their parents aren’t closely related. I share no completely identical DNA with my Ferverda cousin, so no segments are painted dark purple.

23andMe and Ancestry Maps Show Where Your Matches Live

Another reason that adoptees and people searching for birth parents or unknown relatives like 23andMe is because of the map function.

After clicking on DNA Relatives, click on the Map function at the top of the page which displays the following map.

First Steps 23andMe map

click to enlarge

This isn’t a map of where your matches ancestors lived, but is where your matches THEMSELVES live. Furthermore, you can zoom in, click on the button and it displays the name of the individual and the city where they live or whatever they entered in the location field.

First Steps 23andMe your location on map.png

I entered a location in my profile and confirmed that the location indeed displays on my match’s maps by signing on to another family member’s account. What I saw is the display above. I’d wager that most testers don’t realize that their home location and photo, if entered, is being displayed to their matches.

I think sharing my ancestors’ locations is a wonderful, helpful, idea, but there is absolutely no reason whatsoever for anyone to know where I live and I feel it’s stalker-creepy and a safety risk.

First Steps 23andMe questions.png

If you enter a location in this field in your profile, it displays on the map.

If you test with 23andMe and you don’t want your location to display on this map to your matches, don’t answer any question that asks you where you call home or anything similar. I never answer any questions at 23andMe. They are known for asking you the same question repeatedly, in multiple locations and ways, until you relent and answer.

Ancestry has a similar map feature and they’ve also begun to ask you questions that are unrelated to genealogy.

Ancestry Map Shows Where Your Matches Live

At Ancestry, when you click to see your DNA matches, look to the right at the map link.

First Steps Ancestry map link.png

By clicking on this link, you can see the locations that people have entered into their profile.

First Steps Ancestry match map.png

As you can see, above, I don’t have a location entered and I am prompted for one. Note that Ancestry does specifically say that this location will be shown to your matches.

You can click on the Ancestry Profile link here, or go to your Personal Profile by click the dropdown under your user name in the upper right hand corner of any page.

This is important because if you DON’T want your location to show, you need to be sure there is nothing entered in the location field.

First Steps Ancestry profile.png

Under your profile, click “Edit.”

First Steps Ancestry edit profile.png

After clicking edit, complete the information you wish to have public or remove the information you do not.

First Steps Ancestry location in profile.png

Sometimes Your Answer is a Little More Complicated

This is a First Steps article. Sometimes the answer you seek might be a little more complicated. That’s why there are specialists who deal with this all day, everyday.

What issues might be more complex?

If you’re just starting out, don’t worry about these things for now. Just know when you run into something more complex or that doesn’t make sense, I’m here and so are others. Here’s a link to my Help page.

Getting Started

What do you need to get started?

  • You need to take a DNA test, or more specifically, multiple DNA tests. You can test at Ancestry or 23andMe and transfer your results to both Family Tree DNA and MyHeritage, or you can test directly at all vendors.

Neither Ancestry nor 23andMe accept uploads, meaning other vendors tests, but both MyHeritage and Family Tree DNA accept most file versions. Instructions for how to download and upload your DNA results are found below, by vendor:

Both MyHeritage and Family Tree DNA charge a minimal fee to unlock their advanced features such as chromosome browsers and ethnicity if you upload transfer files, but it’s less costly in both cases than testing directly. However, if you want the MyHeritage DNA plus Health or the Family Tree DNA Y DNA or Mitochondrial DNA tests, you must test directly at those companies for those tests.

  • It’s not required, but it would be in your best interest to build as much of a tree at all three vendors as you can. Every little bit helps.

Your first tree-building step should be to record what your family knows about your grandparents and great-grandparents, aunts and uncles. Here’s what my first step attempt looked like. It’s cringe-worthy now, but everyone has to start someplace. Just do it!

You can build a tree at either Ancestry or MyHeritage and download your tree for uploading at the other vendors. Or, you can build the tree using genealogy software on your computer and upload to all 3 places. I maintain my primary tree on my computer using RootsMagic. There are many options. MyHeritage even provides free tree builder software.

Both Ancestry and MyHeritage offer research/data subscriptions that provide you with hints to historical documents that increase what you know about your ancestors. The MyHeritage subscription can be tried for free. I have full subscriptions to both Ancestry and MyHeritage because they both include documents in their collections that the other does not.

Please be aware that document suggestions are hints and each one needs to be evaluated in the context of what you know and what’s reasonable. For example, if your ancestor was born in 1750, they are not included in the 1900 census, nor do women have children at age 70. People do have exactly the same names. FindAGrave information is entered by humans and is not always accurate. Just sayin’…

Evaluate critically and skeptically.

Ok, Let’s Go!

When your DNA results are ready, sign on to each vendor, look at your matches and use this article to begin to feel your way around. It’s exciting and the promise is immense. Feel free to share the link to this article on social media or with anyone else who might need help.

You are the cumulative product of your ancestors. What better way to get to know them than through their DNA that’s shared between you and your cousins!

What can you discover today?

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Disclosure

I receive a small contribution when you click on some of the links to vendors in my articles. This does NOT increase the price you pay but helps me to keep the lights on and this informational blog free for everyone. Please click on the links in the articles or to the vendors below if you are purchasing products or DNA testing.

Thank you so much.

DNA Purchases and Free Transfers

Genealogy Services

Genealogy Research

Full or Half Siblings?

Many people are receiving unexpected sibling matches. Everyday on social media, “surprises” are being reported so often that they are no longer surprising – unless of course you’re the people directly involved and then it’s very personal, life-altering and you’re in shock. Staring at a computer screen in stunned disbelief.

Conversely, sometimes that surprise involves people we already know, love and believe to be full siblings – but autosomal DNA testing casts doubt.

If your sibling doesn’t match at all, download your DNA files and upload to another company to verify. This step can be done quickly.

Often people will retest, from scratch, with another company just for the peace of mind of confirming that a sample didn’t get swapped. If a sample was swapped, then another unknown person will match you at the sibling level, because they would be the one with your sibling’s kit. It’s extremely rare, but it has happened.

If the two siblings aren’t biologically related at all, we need to consider that one or both might have been adopted, but if the siblings do match but are predicted as half siblings, the cold fingers of panic wrap themselves around your heart because the ramifications are immediately obvious.

Your full sibling might not be your full sibling. But how can you tell? For sure? Especially when minutes seem like an eternity and your thoughts are riveted on finding the answer.

This article focuses on two tools to resolve the question of half versus full siblingship, plus a third safeguard.

Half Siblings Versus Step-Siblings

For purposes of clarification, a half sibling is a sibling you share only one parent with, while a step-sibling is your step-parent’s child from a relationship with someone other than your parent. Your step-parent marries your parent but is not your parent. You are not genetically related to your step-siblings unless your parent is related to your step-parent.

Parental Testing

Ideally two people who would like to know if they are full or half siblings would have both parents, or both “assumed” parents to compare their results with. However, life is seldom ideal and parents aren’t always available. Not to mention that parents in a situation where there was some doubt might be reluctant to test.

Furthermore, you may elect NOT to have your parents test if your test with your sibling casts doubt on the biological connections within your family. Think long and hard before exposing family secrets that may devastate people and potentially destroy existing relationships. However, this article is about the science of confirming full versus half siblings, not the ethics of what to do with that information. Let your conscience be your guide, because there is no “undo” button.

Ranges Aren’t Perfect

The good news is that autosomal DNA testing gives us the ability to tell full from half-siblings by comparing the siblings to each other, without any parent’s involvement.

Before we have this discussion, let me be very clear that we are NOT talking about using these tools to attempt to discern a relationship between two more distant unknown people. This is only for people who know, or think they know or suspect themselves to be either full or half siblings.

Why?

Because the ranges of the amount of DNA found in people sharing close family relationships varies and can overlap. In other words, different degrees of relationships can be expected to share the same amounts of DNA. Furthermore, except for parents with whom you share exactly 50% of your autosomal DNA (except males don’t share their father’s X chromosome), there is no hard and fast amount of DNA that you share with any relative. It varies and sometimes rather dramatically.

The first few lines of this Relationship Chart, from the 2016 article Concepts – Relationship Predictions, shows both first and second degree relationships (far right column).

Sibling shared cM chart 2016.png

You can see that first degree relations can be parent/child, or full siblings. Second degree relationships can be half siblings, grandparents, aunt/uncle or niece/nephew.

Today’s article is not about how to discern an unknown relation with someone, but how to determine ONLY if two people are half or full siblings to each other. In other words, we’re only trying to discern between rows two and three, above.

As more data was submitted to Blaine Bettinger’s Shared cM Project, the ranges changed as we continued to learn. Blaine’s 2017 results were combined into a useful visual tool at DNAPainter, showing various relationships.

Sibling shared cM DNAPainter.png

Note that in the 2017 version of the Shared cM Project, the high end of the half sibling range of 2312 overlaps with the low end of the full sibling range of 2209 – and that’s before we consider that the people involved might actually be statistical outliers. Outliers, by their very definition are rare, but they do occur. I have seen them, but not often. Blaine wrote about outliers here and here.

Full or Half Siblings?

So, how to we tell the difference, genetically, between full and half siblings?

There are two parts to this equation, plus an optional third safeguard:

  1. Total number of shared cM (centiMorgans)
  2. Fully Identical Regions (FIR) versus Half Identical Regions (HIR)

You can generally get a good idea just from the first part of the equation, but if there is any question, I prefer to download the results to GedMatch so I can confirm using the second part of the equation too.

The answer to this question is NOT something you want to be wrong about.

Total Number of Shared cM

Each child inherits half of each parent’s DNA, but not the same half. Therefore, full siblings will share approximately 50% of the same DNA, and half siblings will share approximately 25% when compared to each other.

You can see the differences on these charts where percentages are converted into cM (centiMorgans) and on the 2017 combined chart here.

I’ve summarized full and half siblings’ shared cMs of DNA from the 2017 chart, below.

Relationship Average Shared cM Range of Shared cM
Half Siblings 1,783 1,317 – 2,312
Full Siblings 2,629 2,209 – 3,394

Fully Identical and Half Identical Regions

Part of the DNA that full siblings inherit will be the exact same DNA from Mom and Dad, meaning that the siblings will match at the same location on their DNA on both Mom’s strand of DNA and Dad’s strand of DNA. These sections are called Fully Identical Regions, or FIR.

Half siblings won’t fully match, except for very small slivers where the nucleotides just happen to be the same (identical by chance) and that will only be for very short segments.

Half siblings will match each other, but only one parent’s side, called Half Identical Regions or HIR.

Roughly, we expect to see about 25% of the DNA of full siblings be fully identical, which means roughly half of their shared DNA is inherited identically from both parents.

Understanding the Concept of Half Identical Versus Fully Identical

To help understand this concept, every person has two strands of DNA, one from each parent. Think of two sides of a street but with the same addresses on both sides. A segment can “live” from 100-150 Main Street, er, I mean chromosome 1 – but you can’t tell just from the address if it’s on Mom’s side of the street or Dad’s.

However, when you match other people, you’ll be able to differentiate which side is which based on family members from that line and who you match in common with your sibling. This an example of why it’s so important to have close family members test.

Any one segment on either strand being compared between between full siblings can:

  • Not match at all, meaning the siblings inherited different DNA from both parents at this location
  • Match on one strand but not the other, meaning the siblings inherited the same DNA from one parent, but different DNA from the other. (Half identical.)
  • Match identically on both, meaning the siblings inherited exactly the same DNA in that location from both parents. (Fully identical.)

I created this chart to show this concept visually, reflecting the random “heads and tails” combination of DNA segments by comparing 4 sets of full siblings with one another.

Sibling full vs half 8 siblings arrows

This chart illustrates the concept of matching where siblings share:

  • No DNA on this segment (red arrow for child 1 and 2, for example)
  • Half identical regions (HIR) where siblings share the DNA from one parent OR the other (green arrow for child 1 and 2, for example, where the siblings share brown from mother)
  • Fully identical regions (FIR) where they share the same segment from BOTH parents so their DNA matches exactly on both strands (black boxed regions)

If a region isn’t either half or fully identical, it means the siblings don’t match on that piece of DNA at all. That’s to be expected in roughly 50% of the time for full siblings, and 75% of the time for half siblings. That’s no problem, unless the siblings don’t match at all, and that’s entirely different, of course.

Let’s look at how the various vendors address half versus full siblings and what tools we have to determine which is which.

Ancestry

Ancestry predicts a relationship range and provides the amount of shared DNA, but offers no tools for customers to differentiate between half versus full siblings. Ancestry has no chromosome browser to facilitate viewing DNA matches but shared matches can sometimes be useful, especially if other close family members have tested.

Sibling Ancestry.png

Update 4-4-2019 – I was contacted by a colleague who works for an Ancestry company, who provided this information: Ancestry is using “Close Family” to designate avuncular, grandparent/grandchild and half-sibling relationships. If you see “Immediate Family “the relationship is a full sibling.

Customers are not able to view the results for ourselves, but according to my colleague, Ancestry is using FIRs and HIRs behind the scenes to make this designation. The Ancestry Matching White Paper is here, dating from 2016.

If Ancestry changes their current labeling in the future, this may not longer be exactly accurate. Hopefully new labeling would provide more clarity. The good news is that you can verify for yourself at GedMatch.

A big thank you to my colleague!

MyHeritage

MyHeritage provides estimated relationships, a chromosome browser and the amount of shared DNA along with triangulation but no specific tool to determine whether another tester is a full or half sibling. One clue can be if one of the siblings has a proven second cousin or closer match that is absent for the other sibling, meaning the siblings and the second cousin (or closer) do not all match with each other.

Sibling MyHeritage.png

Family Tree DNA

At Family Tree DNA, you can see the amount of shared DNA. They also they predict a relationship range, include a chromosome browser, in common matching and family phasing, also called bucketing which sorts your matches into maternal and paternal sides. They offer additional Y DNA testing which can be extremely useful for males.

Sibling FamilyTreeDNA.png

If the two siblings in question are male, a Y DNA test will shed light on the question of whether or not they share the same father (unless the two fathers are half brothers or otherwise closely related on the direct paternal line).

Sibling advanced matches.png

FamilyTreeDNA provides Advanced Matching tools that facilitate combined matching between Y and autosomal DNA.

Sibling bucketing both.png

FamilyTreeDNA’s Family Finder maternal/paternal bucketing tool is helpful because full siblings should be assigned to “both” parents, shown in purple, not just one parent, assuming any third cousins or closer have tested on both sides, or at least on the side in question.

As you can see, on the test above, the tester matches her sister at a level that could be either a high half sibling match, or a low full sibling match. In this case, it’s a full sibling, not only because both parents tested and she matched, but because even before her parents tested, she was already bucketed to both sides based on cousins who had tested on both the maternal and paternal sides of the family.

GedMatch

GedMatch, an upload site, shows the amount of shared DNA as well. Select the One-to-One matching and the “Graph and Position” option, letting the rest of the settings default.

Sibling GedMatch menu.png

GedMatch doesn’t provide predicted relationship ranges as such, but instead estimates the number of generations to the most recent common ancestor – in this case, the parents.

Sibling GedMatch total.png

However, GedMatch does offer an important feature through their chromosome browser that shows fully identical regions.

To illustrate, first, I’m showing two kits below that are known to be full siblings.

The green areas are FIR or Fully Identical Regions which are easy to spot because of the bright green coloring. Yellow indicate half identical matching regions and red means there is no match.

Sibling GedMatch legend.png

Please note that this legend varies slightly between the legacy GedMatch and GedMatch Genesis, but yellow, green, purple and red thankfully remain the same. The blue base indicates an entire region that matches, while the grey indicates an entire region not considered a match..

Sibling GedMatch FIR.png

Fully identical green regions (FIR) above are easy to differentiate when compared with half siblings who share only half identical regions (HIR).

The second example, below, shows two half-siblings that share one parent.

Sibling GedMatch HIR.png

As you can see, there are slivers of green where the nucleotides that both parents contributed to the respective children just happen to be the same for a very short distance on each chromosome. Compared to the full sibling chart, the green looks very different.

The half-sibling small green segments are fully identical by chance or by population, but not identical by descent which would mean the segments are identical because the individuals share both parents. These two people don’t share both parents.

The fully identical regions for full siblings are much more pronounced, in addition to full siblings generally sharing more total DNA.

GedMatch is the easiest and most useful site to work with for determining half versus full siblings by comparing HIR/FIR. I wrote instructions for downloading your DNA from each of the testing vendors at the links below:

Twins

Fraternal twins are the same as regular siblings. They share the same space for 9 months but are genetically siblings. Identical twins, on the other hand, are nearly impossible to tell apart genetically, and for all intents and purposes cannot be distinguished in this type of testing.

Sibling GedMatch identical twin.png

Here’s the same chart for identical twins.

23andMe

23andMe also provides relationship estimates, along with the amount of shared DNA, a chromosome browser that includes triangulation (although they don’t call it that) and a tool to identify full versus half identical regions. 23andMe does not support trees, a critical tool for genealogists.

Unfortunately, 23andMe has become the “last” company that people use for genealogy. Most of their testers seem to be seeking health information today.

If you just happen to have already tested at 23andMe with your siblings, great, because you can use these tools. If you have not tested at 23andMe, simply upload your results from any vendor to GedMatch.

At 23andMe, under the Ancestry, then DNA Relatives tabs, click on your sibling’s match to view genetic information, assuming you both have opted into matching. If you don’t match your sibling, PLEASE be sure you BOTH have completely opted in for matching. I can’t tell you how many panic stricken siblings I’ve coached who weren’t both opted in to matching. If you’re experiencing difficulty, don’t panic. Simply download both people’s files to GedMatch for an easier comparison. You can find 23andMe download instructions here.

Sibling 23andMe HIR.png

Scrolling down, you can see the options for both half and completely identical segments on your chromosomes as compared to your match. Above,  my child matches me completely on half identical regions. This makes perfect sense, of course, because my father and my child’s father are not the same person and are not related.

Conversely, this next match is my identical twin whom I match completely identically on all segments.

Sibling 23andMe FIR.png

Confession – I don’t have an identical twin. This is actually my V3 test compared with my V4 test, but these two tests are in essence identical twin tests.

Unusual Circumstances

The combination of these two tools, DNA matching and half versus fully identical regions generally provides a relatively conclusive answer as to whether two individuals are half or full siblings. Note the words generally and relatively.

There are circumstances that aren’t as clear cut, such as when the father of the second child is a brother or other close relative of the first child’s father – assuming that both children share the same mother. These people are sometimes called three quarters siblings or niblings.

In other situations, the parents are related, sometimes closely, complicating the genetics.

These cases tend to be quite messy and should be unraveled with the help of a professional. I recommend www.dnaadoption.com (free unknown parent search specialists) or Legacy Tree Genealogists (professional genealogists.)

The Final SafeGuard – Just in Case

A third check, should any doubt remain about full versus half siblings, would be to find a relative that is a second cousin or closer on the presumed mother’s side and one on the presumed father’s side, and compare autosomal results of both relatives to both siblings.

There has never been a documented case of second cousins or closer NOT matching each other. I’m unclear about second cousins once removed, or half second cousins, but about 10% of third cousins don’t match. To date, second cousins (or closer) who didn’t match, didn’t match because they weren’t really biological second cousins.

If the two children are full siblings meaning the biological children of both the presumed parents, both siblings will match the 2nd cousin or closer on the mother’s side AND the 2nd cousin or closer on the father’s side as well. If they are not full siblings, one will match only on the second cousin on the common parent’s side.

You can see in the example below that Child 1 and Child 2, full siblings, match both Hezekiah (green), a second cousin from the father’s side, as well as Susan (pink), a second cousin from the mother’s side.

Sibling both sides matching.png

If one of the two children only matches one cousin, and not the other, then the person who doesn’t match the cousin from the father’s side, for example, is not related to the father – although depending on the distance of the relationship, I would seek an additional cousin to test through a different child – just in case.

You can see in the example below that Child 2 matches both Hezekiah (green) and Susan (pink), but Child 1 only matches Susan (pink), from the mother’s side, meaning that Child 1 does not descend from John, so isn’t the child of the Presumed Father (green).
Sibling both sides not matching.png

If neither child matches Hezekiah, that’s a different story. You need to consider the possibility of one of the following:

  • Neither child is the child of the Presumed Father, and could potentially be fathered by different men
  • A break occurred in the genetic line someplace between John and Hezekiah or between John and the Presumed Father.

In other words, the only way this safeguard works as a final check is if at least ONE of the children matches both presumed parents’ lines with a second cousin or closer.

And yes, these types of “biological lineage disruptions” do occur and much more frequently that first believed.

In the End

You may not need this safeguard check when the first and second methodologies, separately or together, are relatively conclusive. Sometimes these decisions about half versus full siblings incorporate non-genetic situational information, but be careful about tainting your scientific information with confirmation bias – meaning unintentionally skewing the information to produce the result that you might desperately want.

When I’m working with a question as emotionally loaded as trying to determine whether people are half or full siblings, I want every extra check and safeguard available – and you will too. I utilize every tool at my disposal so that I don’t inadvertently draw the wrong conclusion.

I want to make sure I’ve looked under every possible rock for evidence. I try to disprove as much as I try to prove. The question of full versus half siblingship is one of the most common topics of the Quick Consults that I offer. Even when people think they know the answer, it’s not uncommon to ask an expert to take a look to confirm. It’s a very emotional topic and sometimes we are just too close to the subject to be rational and objective.

Regardless of the genetic outcome, I hope that you’ll remember that your siblings are your siblings, your parents are your parents (genetic or otherwise) and love is love – regardless of biology. Please don’t lose the compassionate, human aspect of genealogy in the fervor of the hunt.

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Disclosure

I receive a small contribution when you click on some (but not all) of the links to vendors in my articles. This does NOT increase the price you pay but helps me to keep the lights on and this informational blog free for everyone. Please click on the links in the articles or to the vendors below if you are purchasing products or DNA testing.

Thank you so much.

DNA Purchases and Free Transfers

Genealogy Services

Genealogy Research