Or, in other words, Y not…
For once, the answer isn’t “it depends.”
The answer to reader Hala’s question is a flat out simple no.
The question: “Is Y-DNA testing the most certain for determining if two brothers share the same father??”
And the answer to that is no.
Because of the limits to the test.
YDNA, remember, is the kind of DNA that only males have and that’s passed down from father to son to son (and so on) largely unchanged through the generations, so it’s “used to explore a man’s patrilineal or direct father’s-line ancestry”.1
Now… read that again. And pay particular attention to this part: passed down from father to son to son (and so on) largely unchanged through the generations.
So let’s take a family chart of males only that we think should look like this:
Our theory here is that B1 is the father of B2a and B2b — the two brothers we think should have the same father.
But here’s the hitch.
All of the people on this chart should share the same YDNA.
And it could very well be exactly the same YDNA — without any changes at all.
Now think about that for a minute.
If A1 and C1 have exactly the same YDNA as B1, how can this test tell us that B1 is in fact the father of the two brothers B2a and B2b — rather than A1 being one boy’s father and B1 being the father of the other? Or B1 and C1? Or A1 and C1? Or just A1 as the father of both or just C1 for both?
It can’t even rule out A2 or C2, or — for that matter — A or B or C or even Common Ancestor, if the ages allow.
This test can’t tell us that our theory, shown in this chart, is right.
It can only tell us the two brothers share a common male ancestor, and how far back that common male ancestor might be is a matter of the odds depending on how extensive the test is that we paid for — the number of markers. And here’s what the main company that does genealogical YDNA testing, Family Tree DNA, tells us:
• If the brothers tested only 37 markers, a perfect 37/37 match would indicate that “they share a common male ancestor. Their relatedness is extremely close with the common ancestor predicted, 50% of the time, in five generations or less and over a 95% probability within eight generations.”2
• If the brothers each took the 67-marker test, then a perfect 67/67 match tells us they “share a common male ancestor … with the common ancestor predicted, 50% of the time, in three generations or less and with a 90% probability within five generations.”3
• And if they opted for the genealogical-top-of-the-line 111-marker test, a perfect 111/111 match indicates “a very close or immediate relationship. Most exact matches are 3rd cousins or closer, and over half are related within two generations (1st cousins).”4
In other words, even a perfect match in a genealogically-top-flight YDNA test only tells us the two boys are related through a common male ancestor. It still can’t tell us they’re brothers who share the same father.
To get that kind of information — to show that they’re brothers and not cousins — we need to use a different kind of DNA test: the autosomal DNA test.5
An autosomal DNA test will disclose whether two people are likely siblings (sharing both parents) or half-siblings (sharing one parent). Testing the two brothers and the suspected father will definitively answer the parent-child question for each brother.
If the suspected father can’t or won’t be tested, then testing with a company that provides a full chromosome browser (Family Tree DNA and 23andMe) will be best, since looking at the overall results including the X-chromosome will help suggest whether the one shared parent is the father or the mother. That’s because males only inherit an X-chromosome from their mothers, so shared X-DNA between two males can only have come from a shared maternal line.6
Hala also wanted to know if the raw data files produced by the testing would be legally binding — meaning, I suspect, both admissible in court and considered conclusive of a relationship.
Easy answer and again a flat out simple no. Consumer DNA tests done under uncontrolled circumstances don’t have a chain of custody: the evidence that links the particular person to that particular sample tested by that particular laboratory. So even if a court chose to allow evidence of a test with supporting testimony by the test-taker, it’s highly unlikely that it would be considered conclusive for that reason alone.
There are, you see, limits to the test.
Cite/link to this post: Judy G. Russell, “Limits to the test,” The Legal Genealogist (https://www.legalgenealogist.com/blog : posted 28 Mar 2021).
- ISOGG Wiki (http://www.isogg.org/wiki), “Y chromosome DNA tests,” rev. 30 Oct 2020. ↩
- “If two men share a surname, how should the genetic distance at 37 Y-chromosome STR markers be interpreted?,” Family Tree DNA Learning Center (https://learn.familytreedna.com/ : accessed 28 Mar 2021). ↩
- [2. “If two men share a surname, how should the genetic distance at 67 Y-chromosome STR markers be interpreted?,” Family Tree DNA Learning Center (https://learn.familytreedna.com/ : accessed 28 Mar 2021). ↩
- “If two men share a surname, how should the genetic distance at 111 Y-chromosome STR markers be interpreted?,” Family Tree DNA Learning Center (https://learn.familytreedna.com/ : accessed 28 Mar 2021). ↩
- See ISOGG Wiki (http://www.isogg.org/wiki), “Autosomal DNA,” rev. 21 Oct 2020. ↩
- See generally ISOGG Wiki (http://www.isogg.org/wiki), “X-chromosome testing,” rev. 3 Mar 2021. ↩