Site icon DNAeXplained – Genetic Genealogy

DNA Beginnings: What is a Match?

Roberta Estes

Before we evaluate matches at each of the four major vendors, FamilyTreeDNA, MyHeritage, Ancestry and 23andMe, let’s discuss what a DNA match is, what it means, and what it does NOT mean.

A Match to Another Person

Each of the four major vendors, but not some other vendors, provide matches to you and other individuals in their database.

This example from FamilyTreeDNA shows my mother’s match list listing me as her closest match, along with a kit I uploaded from Ancestry when I was recently updating upload/download article instructions for my readers. You don’t need to upload multiple kits to vendors.

Every vendor’s match list looks different, as is the information they provide. We will cover each vendor’s match list individually in future articles in this DNA Beginnings series.

Each vendor has different criteria for matching, but in essence, using that vendor’s match criteria – your DNA and the DNA of a person you match are identical on a section of DNA of a vendor-defined length.

Each of those vendors identifies the people who match each other and opt-in to matching in one way or another,

When you sign on to your account at each vendor, you’ll see a match list. Each of those people on that list match your DNA:

Each vendor has their own thresholds and internal algorithms that define matches. For example, a match of 8 cM with 1500 SNPs refers to both the length of the match (cM) and the density of locations within that segment of DNA that match between two people. Only matches above each vendor’s threshold appear on your match list.

Matches smaller than or beneath those vendor thresholds are considered less likely to be valid matches, so are excluded and do not appear on your match list.

Imputation Affects Matching

Different vendors test their customers’ DNA on different DNA chips:

In other words, just because two vendors both test 700,000 locations doesn’t mean they test the same 700,000 locations.

Even the same vendor will, over time, implement different DNA testing chips or modify the SNP locations tested on the same chip.

These different chips, chip versions and SNP locations are not fully compatible with each other, so the vendors use a technique known as imputation to level the playing field between non-identical files.

This is particularly relevant for vendors that accept uploads from other vendors.

In this example, we have 3 vendors and 10 different SNPs, or DNA locations.

Therefore only 6 locations, 3-8, were “common” between those two different chips used by the same vendor.

There are only 4 locations out of 10 tested by all the vendors’ chips.

If the vendor’s match criteria is that 10 locations in a row must match, then none of these people will match each other.

Sometimes differences occur because of chip differences, and sometimes a difference occurs because a location doesn’t read well for some reason.

In order to compensate for the differences in DNA locations tested/reported, a technique called imputation is widely used.

Imputation uses scientific probability techniques to fill in the blanks based on DNA that typically neighbors or “travels with” the nucleotides or DNA values, (T, A, C or G), found in the customer being tested.

Imputation allows all of those blanks to be filled in for all customers for each of those 10 locations, assuming the “missing DNA” is close to tested DNA locations.

It’s thanks to imputation that customers can download their raw DNA files from one vendor and upload to another for matching, even though the vendors don’t use the same exact chip.

Sometimes imputation is incorrect. Matching can be affected in both directions, meaning that some people will be on each other’s match lists who actually don’t match on a particular segment. Others would actually match if all of those locations were tested.

The highest quality matches are between people who tested at the same vendor, on the same chip or at two different vendors who use exactly the same chip. However, that’s often not possible and isn’t within the control of the customer.

False Positive Matches

This translates to, “You’re a match but not really” and is a headache for genealogists.

False positive matches show as a match between two people on their match lists, but they aren’t actually valid matches for genealogy.

For purposes of these examples, presume that each of these matches exceeds the vendor’s match criteria so would be shown on your match list.

In our example, Person 1 and Person 2 match at all 10 locations, so they would appear on each other’s match lists.

However, if we could see the DNA of Person 2’s parents, we would see that Person 2 DOES match Person 1, but is NOT a valid match. Person 3 inherited the first 5 DNA locations from their mother and the second 5 DNA locations from their father.

While Person 2 technically is a match to Person 1, they aren’t a legitimate match because the segment of DNA that matches does not descend from the same parent. This means that the DNA did not descend in one piece from ONE ancestor, but clearly descended in pieces from two ancestors – one maternal and one paternal.

Therefore a technical match that is not a genealogical match because the DNA is inherited in part from both parents is known as a false positive and is said to be Identical by Chance, or IBC. You can read about IBC matches here.

False Negative Matches

A false negative match is just the opposite. False negatives occur when two people are NOT reported on each other’s match lists when they actually would match if all of the DNA at the various required locations were tested, read, and reported accurately. In other words, if imputation were not necessary.

For example, if one vendor reports matches at 6 cM and above, and a second vendor reports matches at 8 cM and above, the same two people who match at 7 cM will match at the first vendor, but not at the second.

The only way you would ever know about a false negative match, because they aren’t reported, is if you simply happen to match at a vendor who allows smaller thresholds.

Also, keep in mind that each vendor creates their own imputations algorithms, so two different vendors using imputation on the same file may produce different results.

Determining Valid Matches

So, how might you determine which matches are actually valid matches?

That’s a great question.

There are useful “hints:”

Keep in mind that each matching segment must be confirmed separately, and not every vendor shares the locations of the segments that match.

So What Is a Match?

A match is not a guarantee that you share a common ancestor unless you are matching to close relatives. You won’t match a close relative if the match is not valid.

What About You?

What is your plan to verify that your matches are valid?

Have your parents tested their DNA? Either of both parents?

If so, ask for your parents to upload their DNA with you to each vendor where you upload your own results.

At each vendor, you’ll have different matches. That’s exactly why we fish in multiple ponds.

I always work with my closest matches first, because I’m the most likely to be able to easily identify our common ancestor.

Locate your closest known relatives from both your mother’s side and your father’s side at each vendor. These people will be extremely helpful for our next article about shared matches.

_____________________________________________________________

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 Products and Services

Books

Genealogy Research

Exit mobile version