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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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