I was reading a book wherein two full brothers had married two full sisters (one each, of course). One couple had a boy child, the other couple had a girl child. These two grew up together and were very close to the point where there might have been some hanky-panky when they were teenagers. Well, even first cousins might think this is OK.
Then they find out that they are actually half-siblings, as the father of the boy was also the father of the girl (apparently an open secret to the parents, but unknown to the children). So they panic and feel suicidal, due to incest taboos and shame and stuff. Now my questions:
Without the shared father, wouldn’t these two have been closer than cousins genetically? Normal cousins only share DNA on one side of their family line; these shared DNA on both sides. It seems to me, as nearly as I can wrap my head around it, that they would have been more like half-sibs anyway, considering the shared DNA situation.
After the discovery of the girl’s true father, it seems again like they would have been closer than half-sibs, but not as close as full sibs. Sort of 3/4 sibs.
Have I got this right, or are my calculations too naive?
There are a couple of useful ways to measure these kinds of relationships: one is the expected value of shared genes and the other is the number of shared ancestors in the immediate family tree. For example - siblings have an expected value of 50% (with a theoretical range of 0-100% of their genes in common) of their genes in common and share 100% of the same grandparents.
Regular cousins have an expected value of 25% of their genes in common (with a theoretical range of 0-50%) and two out of four grandparents in common.
So, in the case of two sisters marrying two brothers, then yes – even without the added twist of one man fathering both kids, the relationship is closer than usual for cousins:
The expected value of genes in common is the same as for siblings, rather than for cousins. Likewise the number of shared grandparents is 4 instead of 2.
With 1 man fathering offspring by sisters, the number of shared grandparents is unchanged – you still have 4 shared grandparents. But the expected number of genes in common is … okay, I’m gonna guess it is 37.5 percent. (25% from dad and 12.5% from the sister-moms.) Someone who wants to think this through for real instead of intuit it is welcome to correct me!
Typical cousins would be 12.5%. Two brothers have 50% of their genes in common. When they have children, they contribute half the genes so each child has 25% of their genes from the common pool. But for each gene from the common pool in the child, there’s only a 50% chance that the cousin got the same one. So you have to halve the 25% one more time.
In the case of two brothers marrying two sisters it’s 25% as each parent is contributing an average of 12.5% of genes found in the child’s cousin. So they’re genetically as related as half-siblings. 37.5% is correct for one father having kids with two sisters. 37.5% is 3/4 of 50%. So the OP was correct about how genetically related the kids would be in both situations. I think. I am not a geneticist or a biologist but I do share around 50% of my genes with one.
Oops, not sure where my brain was when I said 25% for cousins. But it is not correct to say that two brothers have 50% of their genes in common. They could theoretically have anywhere from 0 to 100% in common - 50% is just the expected value.