Understanding the Y match

We get Y with a little help from our friends

With all the attention paid to autosomal DNA testing these days — with good reason, since it’s the newest, most complicated, most difficult (and most fun!) DNA test to work with — we sometimes forget that there are other, older, somewhat simpler DNA tests out there that can sometimes answer our questions.

And then again maybe not.

YDNAToday’s reader question is one of those maybe-not situations.

Reader Barb DeMarco is trying to understand YDNA results. And, she asks, “Can knowing the genetic distance from a Y-DNA match, or say 35/37 markers, help to identify at what generation the link is made? What does a 37/37 match mean?”

Let’s start by defining some terms:

YDNA, remember, is the kind of DNA that only men have — it’s contained within the gender-determinative Y chromosome that’s passed down through the generations from father to son to son. So only men can take this test and it provides information on the direct paternal line only.1

• The particular DNA sequences being looked at in YDNA testing are called short tandem repeats (STRs): patterns of DNA code that form sequences and where the sequences are repeated a number of times at those particular locations on the Y chromosome.2

• The markers are the genes or DNA sequences with known locations within a chromosome that make them useful for comparing one person’s DNA with another.3 In the case of YDNA testing, it’s the STRs — those short tandem repeats — in a number of locations (12, 25, 37, 67 or 111 if you’ve tested at Family Tree DNA) that are the markers being reviewed.

Genetic distance is a term used to describe the differences, if any, between the markers reported in one person’s DNA results and those reported in a second person’s results. “A genetic distance of zero means that there are no differences in the two results and there is an exact match.”4 If the genetic distance is small enough, the two people are considered a match, meaning they share a common ancestor.

But which ancestor? How far back? That’s the $64,000 question,5 isn’t it? We’re all out to identify that wondrous, sometimes-mythical-seeming creature called the Most Recent Common Ancestor (or MRCA) — the most recent individual from which those tested are directly descended6 — and to figure out how many generations back that person lived, called the Time to Most Recent Common Ancestor (or TMRCA).7

So Barb’s problem involves looking at the results of a test where 37 markers were looked at, and trying to understand what it means if there’s a 37-for-37 match (which means a genetic distance of zero) and what it means if there’s only a 35-for-37 match (which means a genetic distance of two). And whether that difference can help narrow down the MRCA or the TMRCA.

Family Tree DNA — where most YDNA testing takes place because of its wide range of surname projects — has a really good Frequently Asked Questions section which helps explain what it means to match another person in a YDNA test where the two men share a surname in 12 markers, 25 markers, 37 markers, 67 markers and 111 markers.

For 37 markers, here’s what Family Tree DNA says about genetic distances of 0, 1 and 2, for men who share a surname:8

Genetic Distance Relationship Interpretation
0 Very Tightly Related

A 37/37 match between two men who share a common surname (or variant) means they share a common male ancestor. Their relatedness is extremely close with the common ancestor predicted, 50% of the time, in 5 generations or less and over a 95% probability within 8 generations. Very few people achieve this close level of a match.

All confidence levels are well within the time frame that surnames were adopted in Western Europe.

1 Tightly Related

A 36/37 match between two men who share a common surname (or variant) indicates a close genealogical match. Very few people achieve this close level of a match, and it is within the range of most well-established surname lineages in Western Europe.

It’s most likely that they matched 24/25 or 25/25 on a previous Y-DNA test, and the mismatch will be found within DYS576, DYS570, or CDY.

2 Related

A 35/37 match between two men who share a common surname (or variant) means they share a common male ancestor. The mismatch is likely within the range of most well-established surname lineages in Western Europe.

It is most likely that you matched exactly or closely on previous Y-DNA tests and the mismatch is within DYS439 or DYS385, DYS389i, 389ii, DYS458, DYS459, DYS449, DYS464, DYS576, DYS570, or CDY.

See the problem? It’s a statement of odds. And there are a whole lot of variables that affect the odds.

First and foremost, we need to understand that markers change — the changes are called mutations, but the changes are usually benign, not the three-eyes-two-heads type of mental image we tend to get when we hear the word mutation. And they can change in a single generation: a father may match his own son in only 36 of 37 markers.

Second, we need to understand that some markers change more quickly than others. That’s why you see the references in the chart above to mismatches in certain specific markers. They’re the ones known to change faster than others, and where you’re most likely to see a difference even between father and son.

Third, we need to understand that the marker changes can be random: they can occur in the same place in the YDNA but in different generations or different lines of descent. In my Baker family, for example, there is one marker out of 37 — a fast-changing marker — where two-thirds of our group show one result but fully one third of the men tested have the same different result. You might expect that they’d all trace back to the same son or grandson of our earliest known ancestor. And four of them do descend from a particular son. But the fifth one doesn’t. He descends from a different one.

Finally, whenever we’re dealing with statistics, we have to remember that changing one variable will change the odds. For example, my Uncle David’s YDNA is a 37-for-37 match with another man of the same surname. Assuming that we know they don’t share a common father (one generation), the chances are 89.48% that they share a common ancestor within five generations. Assuming that we know they don’t share a common grandfather (two generations), the chances drop to 83.49% that they share a common ancestor within five generations. And assuming that we know they don’t share a common great grandfather (three generations), the chances are only 74.10% that they share a common ancestor within five generations.9

The odds are different if the match is 35-for-37 markers. Again using my Uncle David and one of his same-surname matches, if we know they don’t share a common father (one generation), the chances are 41.65% that they share a common ancestor within five generations. If we know they don’t share a common grandfather (two generations), the chances drop to 40.13% that they share a common ancestor within five generations. And if we know they don’t share a common great grandfather (three generations), the chances are only 35.76% that they share a common ancestor within five generations.10

So the answer to Barb’s question is a complicated one. We know for sure that a 37-for-37 marker match means the two men are closely related from a genetic perspective. A 35-for-37 match means they are related but not as tightly. But can it tell us anything about how far back they may find the common ancestor?

Sure.

As long as we understand that it’s all a matter of the odds.


SOURCES

  1. See ISOGG Wiki (http://www.isogg.org/wiki), “Y chromosome DNA tests,” rev. 7 Dec 2013.
  2. ISOGG Wiki (http://www.isogg.org/wiki), “Short tandem repeat,” rev. 20 July 2013.
  3. See Emily D. Aulicino, Genetic Genealogy: The Basics and Beyond, Kindle edition (Bloomington, IN : AuthorHouse, 2014), Appendix F: Glossary, “marker.”
  4. ISOGG Wiki (http://www.isogg.org/wiki), “Genetic distance,” rev. 9 Nov 2013.
  5. Wikipedia (http://www.wikipedia.com), “The $64,000 Question,” rev. 24 Jan 2014.
  6. ISOGG Wiki (http://www.isogg.org/wiki), “Most recent common ancestor,” rev. 27 Oct 2013.
  7. Ibid.
  8. If two men share a surname, how should the genetic distance at 37 Y-chromosome STR markers be interpreted?,” Frequently Asked Questions, Family Tree DNA (http://www.familytreedna.com/faq/ : accessed 25 Jan 2014).
  9. FTDNATiP analysis on known kits, names withheld for privacy, Family Tree DNA (http://www.familytreedna.com : accessed 25 Jan 2014).
  10. Ibid.
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11 Responses to Understanding the Y match

  1. Thanks for clarifying these points. People often seem to have great difficulty understanding probabilities. However, in addition to considering the odds we are finding that it is increasingly important to consider the SNP testing status of a match. There have been cases of seemingly close matches at 37 markers, even with the same surname, where the two people have turned out to be in different subclades and hence not related in a genealogical time frame. This is a particular problem with haplogroup R1b1a2. See the ISOGG Wiki article on convergence for the background on this problem:

    http://www.isogg.org/wiki/Convergence

  2. Angie Bush says:

    Family Tree DNA provides a TiP report – (orange button on matches list) that calculates the TMRCA. This report takes into account differences in marker mutation rates, and the number of markers that differ between those in your match list. It can be a useful tool.

    • Judy G. Russell says:

      Hmmm… I probably should have made it clearer that that’s what I used for the comparison of my uncle’s 35-for-37 and 37-for-37 matches — but it’s in the source footnotes!!!

  3. Carolyn Lea says:

    I have a surname back 5 generations (3rd great) on my father’s direct line. One reason I wanted to have my brother test was to possibly identify other surnames that may have been taken when Jews were forced to take surnames about 1812. I did the 67 marker. Was that totally misguided? I sincerely doubt there will be other Schwarzbaums (uncommon name) that have tested so there will not be a surname project.

    I am completely confused as to how one does research with patrilineal names. If you don’t yet know the name of the father I am not sure how you can discover who is the son of the father. But that is another topic.

    • Judy G. Russell says:

      Carolyn, doing a Y test is a good way to check possible other surnames adopted in populations that took surnames late. My cousin Dick, whose paternal line is Ashkenazi, has NO matches to his own surname — but a couple of good matches to one other, which may point the way back to a particular location. The problem with the Ashkenazi population in particular is endogamy (intramarriage within the group). So you’re always dealing with so many more matches than you might in less endogamous populations. The leads can be overwhelming.

      • Carolyn Lea says:

        Thank you. I know. I have so much intermarriage in my own family that I do not even understand when someone says they are only doing their direct line. Even among my mother’s German Lutherans, cousins md cousins.

        But it is good to know I did not waste my money and the results may be helpful.

        I have used the new tool dnagedcom (Don Worth) (http://www.dnagedcom.com/) which allows you to see much more information on a larger group than using the tools at FTDNA. My sister and I have a different group of 2-4th cousin matches on autosomal results which indicates how important it is to test everyone you can.

        I can’t wait to see my brother’s results and see which “deck of cards” he was dealt.

  4. Phil Grass says:

    Sorry to be completely anal Judy, but there is a typo in your references. Item 5 should read the $64,000 question.

  5. Warren Coleman says:

    Glad to hear the reference to a match where the surname is not the same. I have a 37/37 match with a fellow with a different surname. My kin have eight generations on this side of the Atlantic (a fairly complete Tree) and his about five also a good Tree. He says that the answer is in England and I believe that it is a non-paternal event. We will likely never know.

    • Judy G. Russell says:

      Warren, one of the best treatments I’ve found of the different-surname scenario can be found here at the Bowes One-Name Study website.

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