A Staff Report from the Straight Dope Science Advisory Board

If twins marry twins, are the children from the two families cousins--or brothers and sisters?

Dear Straight Dope:

I heard somewhere that if identical twins marry identical twins and they all had children that the children from the two families are actually brothers or sisters. Could this be genetically possible?

Amy, you and I need to discuss sexual reproduction. (Pause for sophomoric giggling from the rest of the SDSAB.) In species that pass down their DNA sexually, the gametes of the father unite with the gametes of the mother. The kicker is, the offspring get exactly half of their DNA from each parent. Which half? Richard Dawkins, in The Blind Watchmaker, says the process isn't exactly a coin flip, but "can be treated as random.... Every sperm produced and every egg produced is unique in terms of the contents of their locations [i.e. the makeup of which nucleotides - or DNA particles - go into which chromosomes]." The upshot of this is that you are exactly 50% related, genetically speaking, to your father and 50% to your mother. Simple probability shows that you are also 50% related to a brother or sister--in other words, the two of you are likely to have 50% of your genetic material in common. Except for an identical twin, to whom you are 100% (except for some typical, meaningless mutations) related.

Now, look at the second generation. I am 50% related to my brother. Our wives are not related. Therefore, my kids get 'diluted' by a factor of 1/2, and are 1/4 related to him. His kids are, of course, 1/4 related to me, and therefore 1/8 related to their first cousins, my kids, all due to the aforementioned probability. Now, what if we were twins? My gametes, which contain exactly 1/2 of my genetic material, would also contain 1/2 the genetic material of someone who shared my DNA. That is, since we are identical in every way, one of us can be freely substituted for the other in this little tree. My kids are still 1/2 related to me, but now you can see that they are also 1/2 related to their uncle! Similarly, his kids are 1/4, rather than 1/8, related to my kids; the inclusion of one set of twins in the family tree doubles the genetic relationship of the parties involved. If our wives are also twins, the 'relatedness' is doubled once again, so that my and my wife's kids would be 1/2 related to my brother's and her sister's. Now, we saw in the last paragraph that brothers and sisters share 50% of their genetic material, and in this special case, our kids, first cousins, would be just as related to each other as if they were siblings.

Of course, this is all the workings of biologists and trained statisticians. If two identical twin sisters have children, while they may be equally related to their kids as to their nieces and nephews, a mom knows which kids she carried in her womb, and which ones she visited after her sister was in labor for sixteen hours. The kids will know which members of the same generation they despised while growing up in the same house with, and which ones they were sent to play with in the other room during the Super Bowl. Finally, even if dad A is genetically identical to dad B, moms A and B will probably not go for the 'free substitution' principle, which I mentioned earlier, in turning their haploid gametes into diploid zygotes, if you get my meaning. This algebraic figuring doesn't really get at what it truly means to be in a family, and 'relatedness' is not equal to 'a relationship', no matter what the DNA tells us. So, to answer your question, no, this genetic 'relatedness' doesn't really mean they are "actually brothers and sisters" by any means except hypothetical.

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