31. The Three Flavors of DNA: Y Chromosome, Autosomal and Mitochondrial Testing

The Three Flavors of DNA: Y Chromosome, Autosomal and Mitochondrial Testing

Editor’s Note: Joe gives a layman’s tutorial on the three DNA tests used in genealogy. The Y chromosome test follows the direct male line, straight back from son to father to grandfather like an arrow shot into the past, and is what he and Jimmy Harte chose. Autosomal testing — I had to ask him to spell it — uses the 22 pairs of non-sex chromosomes and is by far the most popular, but its usefulness fades after about six generations as inherited segments get chopped smaller and smaller. Mitochondrial DNA travels down the female line and, in Joe’s opinion, is the least useful of the three.

Transcript

Speakers: WJB — William J. Bowe; JH — Joseph Hart. Transcribed from the recording; punctuation added and obvious mis-hearings corrected.

JH: Let me quickly explain what the different tests are for DNA, if you’re interested in genealogy. Y chromosome follows the direct male line because only the males have Y chromosomes. So it doesn’t do any good for a female to be interested in the direct male line using Y chromosome, unless they can recruit a brother, a male cousin, an uncle. So the other more popular test, I guess, is autosomal DNA, using the autosomal chromosomes. So you have 22 pairs of chromosomes. The 22 are the autosomal chromosomes.

WJB: And how do you spell autosomal? I’m not sure I understand.

JH: It’s A-U-T-O, like auto, somal, S-O-M-A-L. And somal is kind of a Latin word that refers to basically a root word for the body, is the best I can think of. So these are the chromosomes that are responsible for what you look like as you grow up. They contribute to the traits that you inherit from your parents and a number of other things. And so that’s another test.

The Y chromosome goes for hundreds of years, straight back from son to the father, to the grandfather, to the great-grandfather, just like in a straight line, like you’re shooting an arrow back in the past. It’s very stable over time. If it was like a gene — it’s not exactly like a gene, but there are changes that take place over time, and so it allows you to pinpoint a little bit where in time a Y chromosome match might be from you, or back to a common ancestor. So that’s what Jimmy and I were looking for. So we ended up using the Y chromosome test.

But let me talk a little bit more about the autosomal. So that’s these other 22 pairs of chromosomes in every cell that you have, besides the sex chromosomes, which are the X and the Y. They aren’t called autosomal chromosomes, but the X sometimes is matched up, is tested when you do an autosomal test. So you can use the X chromosome to some degree, can be helpful, but not as much as just looking at the autosomal chromosomes.

And the other DNA test is called the mitochondrial DNA. So the mitochondria is another little element in your cell. It is responsible for some cellular metabolic functions. And it has a little bit of its own DNA. And so that can be tested, but it really isn’t of much use for genealogy unless you’re more interested in your direct female line, because the mitochondrial DNA is passed down by the mother. The mother gives the mitochondrial DNA to both their male and female children, but it basically travels in the female line. But it’s so stable over eons that, in my opinion, of the three tests, it’s probably the least useful for genealogy. And of course, the Y chromosome is only useful if you’re researching your direct male line. So the autosomal test is by far the more popular.

The drawback to autosomal testing is that it’s only useful over about six generations back from the person who is taking the test. Six generations is about fifth cousins. Fourth cousins, I guess. Four great-grandparents back. And even at that, it’s strongest if you’re comparing yourself with siblings or your parents. It’s next strongest if you’re trying to compare yourself with first cousins. It’s the next strongest with second cousins, and so every generation you go back, all the DNA that came down from a common ancestor gets chopped into smaller and smaller bits. And to the point where by the time you get to dividing it up over six generations, the segments are so small that, statistically speaking, they’re of no use for determining who your common ancestor might be. So that has a limit compared to the Y chromosome or the mitochondrial DNA.