50th cousins? Really?

[walkitout]

I'm trying to write this while watching TRMS cover a Florida State state representative who used the phrase "incorporate my uterus" or some variation thereof to illuminate the difference between Republican attitudes towards corporations vs. reproductive choice. It's hard, especially since the leadership went after him on the basis of referring to body parts (hmmmm), and I can't help but think out that uteruses (uteri?) _are_ incorporated. Duh. I mean, _think_ about what "incorporated" means. Part of the body, right? If they _weren't_ incorporated into adult women, this wouldn't be so problematic an issue.

Anyway. As long as I'm talking about reproduction, I really am having my doubts about this whole idea that everyone on the planet is maybe a 50th cousin. I realize that this crossed my horizon of awareness via a wikipedia article on pedigree collapse which quotes a Straight Dope column, but the sourcing seems real (a mid 1980s book by Shoumatoff, _Mountain of Names_). I've ordered a used copy so I can ascertain his reasoning more directly, but on the face of it, it seems incredibly unlikely. Cousins are "count generations back to a shared ancestor and subtract one". That sounds like less than 2000 years to a shared ancestor. I suppose he _might_ mean some idealized zero-pedigree-collapse family trees, figure the amount of genetic divergence that occurs in that time frame, and then translate that back into real family trees. And given how recently the mixing between the Americas and the EurasiaAfrica groups has been, it's just hard hard hard to believe this as a theory.

Maybe he means something else entirely by "cousin".

Comments

I work in a population genetics lab...

[(Anonymous)]

...and while I don't know this to be true for a fact, it seems credible. If, after some thought you still have doubts, let's talk via email or phone.

In the mean time, it may help to remember that because of recombination, after a few generations, the genes at each end of a person's chromosome are from completely different sources. When you produced the ova that resulted in each of your children, odds are very good that each of your pairs of chromosomes "crossed over" during that process, resulting in (for example) T getting the short arm of the chromosome 8 you got from your mom, and the long arm you got from your dad (each of which your parents' bodies recombined from the copies your grandparents passed on to them). I think the number of crossings is an order of magnitude that suggests, to a first approximation, one or two recombinations per chromosome per generation. If you had an oracle who could tell you your and R's common ancestor was, base pair by base pair along your genomes, you would find that it kept changing as you moved along each chromosome. That's not even mentioning that for your autosomes, you get four different ways to pair them up looking for relatives.

AND, that's only considering the genes that survived into your particular genomes. It's very likely that your and R's most recent common ancestor did not pass genes on to either of you, but that shared ancestry still makes you cousins of a particular magnitude.

While I was typing the above, the head of my lab came in. He says to go to the link below and see his talk (in two parts) under the "Evening at the Genome" lecture. He recommends having the PDF copy of the slides. http://evolution.gs.washington.edu/felsenstein.html

-- Elizabeth

Re: I work in a population genetics lab...

[walkitout]

Please don't anyone else waste their time listening to that talk. It does not address the issue at hand. At all. However, I'm going to address the issue at hand in a top level post, because I finally found the information I was looking for.