Fun with Chromosomes and Math

Let’s talk about chromosomes and heritage, and see how much I can remember from biology class a couple years ago.

Humans have 46 chromosomes, which are in 23 pairs, normally numbered from 1 to 23. When you were conceived, you got 23 from your father and 23 from your mother. Which ones you get is random, but you get one from each pair from each parent. So you end up with two #1 chromosomes making up your #1 chromosome pair, and so on down to #23. There’s also something called crossover when the gametes are being created, so the chromosomes a parent passes down are spliced together chunks of the two halves of a pair rather than copies of whole chromosomes, but that doesn’t change the math here.

This is all true for each generation. Your dad’s #1 pair has a chromosome from his father and one from his mother. So on average you’ll get 11.5 chromosomes from your paternal grandfather and 11.5 from your paternal grandmother. The ratio could lean one direction or the other, but it’ll add up to 23. Same thing on your maternal side.

Continuing back, you will average 5.75 chromosomes from each great-grandparent, 2.875 from each great-great-grandparent, 1.4375 from each before that, and 0.71875 from the generation before that. By the time you get back to your great-great-great-great-great grandparents, you have less than a 50% chance of having even a single chromosome’s worth of genes from a particular ancestor. When you go much further back, everyone in the same general racial/ethnic pool (haplogroup) basically has the same gene pool to draw on.

That’s why American Indian groups generally require 18 or 116 Indian heritage for membership. If you go back further than that, everyone might or might not have a little. It’s why dairy cows that are 78 one breed are considered purebred. That other 18 of something else just doesn’t affect much.

And it’s why it doesn’t matter whether Elizabeth Warren has zero or one American Indian ancestors out of 256. The reason the Pocahontas nickname burns her so much that she keeps fighting it is that she built a career on the claim that having American Indian blood gave her special moral authority to lead Americans that she wouldn’t have had without it. That’s pretty gross, really. Whether she was right about her DNA doesn’t change that. “You should vote for me because I have such-and-such blood” is supposed to be an unacceptable argument in 2018, whether you’re lying about the blood or not.

Line of the day from anon: “You have to look so far back that her ancestor may actually be Pocahontas.”


I got trips today, which is always fun. I predicted it here three weeks ago, and articles about it started showing up this weekend. It’s still #HerTurn if she’s still walking around in 2020.