Suppose A and B are brother and sister. They have a son, C. C and B have a son, D. D and B have a son, E, and a daughter F. How genetically related will be the children of E and F, given they do not interbreed?
The simplistic approach is that A and B share 1⁄2 of their (variable) genes by virtue of being siblings, and so their child C will have that shared half, and half of the remainder (i.e. a quarter) will come from B, so C and B share 3⁄4 of their genes. By the same approach, D and B will share 7⁄8 of their genes, and thus E and F will have 7⁄8 shared for certain and 1⁄16 shared by chance, and so their children will share about 15/32ths of their genes, i.e. be about as related as actual siblings.
Thank you. (It is odd how difficult it is to suppose that a random cat is the result of sequential inbreeding—they really look no different than random cats.)
As I understand it, the reason the simplistic approach doesn’t quite work is because the knowledge that a genetic combination produced a functioning adult allows you to update on the total degree of sharing / whether or not any of the ruinous parts were shared.
Suppose A and B are brother and sister. They have a son, C. C and B have a son, D. D and B have a son, E, and a daughter F. How genetically related will be the children of E and F, given they do not interbreed?
(This is actually the history of our cats.)
The simplistic approach is that A and B share 1⁄2 of their (variable) genes by virtue of being siblings, and so their child C will have that shared half, and half of the remainder (i.e. a quarter) will come from B, so C and B share 3⁄4 of their genes. By the same approach, D and B will share 7⁄8 of their genes, and thus E and F will have 7⁄8 shared for certain and 1⁄16 shared by chance, and so their children will share about 15/32ths of their genes, i.e. be about as related as actual siblings.
Thank you. (It is odd how difficult it is to suppose that a random cat is the result of sequential inbreeding—they really look no different than random cats.)
In related news, check out MawBTS’s comment on this Cochran post on inbreeding, on Cleopatra’s ancestry. Yikes.
As I understand it, the reason the simplistic approach doesn’t quite work is because the knowledge that a genetic combination produced a functioning adult allows you to update on the total degree of sharing / whether or not any of the ruinous parts were shared.