There aren’t many words relevant to genealogy that start with X – I don’t suppose many of us encounter the xenodochium (in old English law, an inn allowed by public licence, for the entertainment of strangers) — but there’s one we all need to learn more about.
It’s X-DNA – a kind of oddball variety we don’t use much but which can help us greatly in narrowing down our cousin matches to particular lines of descent.
“…Everybody has X-DNA…”
By definition, it’s the DNA in one of two gender-determinative chromosomes (the other being the Y chromosome). Everybody has X-DNA, but we inherit it in patterns that are different from the rest of our DNA.
A man gets only one X chromosome, from his mother, so his X-DNA is only from his mother’s side of the family. There’s nothing inherited in the X-DNA from his father’s side at all. But when a mother passes on her X, she gives her child a mix of the X-DNA she got from her mother and her father randomly jumbled up in a process called recombination.
“And that’s where the random mixing took place: at his mother’s level”
A woman gets two X chromosomes, one from her mother that’s the same type of random mix and one from her father. And since her father doesn’t have another X chromosome for his X to recombine with when he passes it on, what he gives to his daughter is exactly the same X he got from his mother. And that’s where the random mixing took place: at his mother’s level.
Now think about that for a minute. If two women share the same father, they must share one exact copy of the X-DNA chromosome. A father can only pass on the exact X chromosome he received from his mother. You can already see how to use that, right? If half-sisters share an exact X chromosome, then they share the same father. If they don’t, then they’re half-sisters on their mother’s side.
For men, X-DNA can be used to help figure out cousin matches – the kind of matches we get from an autosomal DNA test like the one from Ancestry DNA or 23andMe or MyHeritage or the Family Finder test from FamilyTreeDNA. Because of the way the X chromosome is handed down, there are fewer ancestors who could possibly have contributed DNA to a man’s X-DNA, and an X-DNA match means that match has to come from only those lines. This chart – produced by THE Genealogy Show UK’s speaker Blaine T. Bettinger – shows in pink and blue the only ancestors that could contribute to a man’s X-DNA.
In my family, we’d lost track of one second great grand-uncle, Charles Baker, son of Martin and Elizabeth (Buchanan) Baker. He seemed to have disappeared after 1854. But a DNA match appeared one day who, we thought, could descend from Charles. And this match was not just an autosomal match, but an X-DNA match to other men we knew descended from Martin and Elizabeth. But if we were right, the X-DNA would have to have travelled down one of the few specific paths for each of them.
Using Blaine’s chart, I coloured in the X-DNA path for known Cousins 1 and 2:
And then I coloured in the X-DNA path for new Cousin 3:
And it works!
Hello there, Uncle Charlie. Welcome back to the family.
X-DNA: we use it because X marks the spot.
Written by Judy G. Russell