inclusive fitness suggests that people who share a greater number of genes with one another — for instance, family members — are more likely to be altruistic towards one another.

but some family members share more genes with each other than others — parents, obviously, definitely share 50% of their genes with each of their kids, so they are really likely to be very altruistic towards their kids. siblings probably share 50% of their genes with one another, so they’re quite likely to be altruistic towards one another

in calculating relatedness between family members, there are some individuals that might seem to be the equivalent of others at first glance, but are in fact somewhat different.

fox, et. al., looked at the differential x-chromosome relatedness of grandmothers to their various grandchildren and found that the level of altruism (determined by looking at child mortality and presence of grandmothers) varied according to how related the grandmas were to each type of grandchild.

from that article:

“Differential X-relatedness between grandmothers and grandchildren

“(i) Paternal grandmother–girl: 50 per cent

“The PGM has two X-chromosomes, and passes one of them to her son. Therefore, any individual gene on her X-chromosome has a 50 per cent likelihood of being inherited by her son. This is the one and only X-chromosome a man has. He passes this chromosome on to his daughter, and so the X-relatedness between a PGM and granddaughter is, just as between a woman and her son, 50 per cent (figure 1).

“(ii) Patneral grandmother–boy: 0 per cent

“Because the only heterosomal relatedness boys have with their fathers is the Y-chromosome, they have no X-relatedness with the PGM.

“(iii) Maternal grandmother–boy and maternal grandmother–girl: 25 per cent

“The MGM has two X-chromosomes, and so any given X-linked gene of hers has a 50 per cent chance of being transmitted to her daughter. Her daughter has one X from the MGM and one X from the maternal grandfather (‘MGF’). She will pass down one of those two X chromosomes to each child, regardless of whether the child is a male or a female. Thus, the transmission likelihood for any particular X-linked gene is 0.5 inheritance from MGMto mother, and 0.5 from mother to child. Therefore, the likelihood of a grandchild inheriting any particular gene fromtheMGMis 25 per cent (0.5  0.5 ¼ 0.25)….

“Although the X-chromosome contains only about 4.4 per cent of our DNA, with its estimated 1529 genes, it contains perhaps approximately 8 per cent of all human genes (Pennisi 2003; NIH 2007; Parang et al. 2008; NCBI 2009a). The dramatic differences in X-relatedness between grandmothers and grandchildren confound the Hamiltonian concept that grandchildren are 25 per cent genetically related to each grandparent. If approximately 92 per cent of our genes are autosomes, then a grandmother shares one-quarter of that, or approximately 23 per cent of her total genes with a grandchild, plus X-relatedness.1 If a grandmother shares no X-chromosome with a grandchild, then their overall genetic relatedness is approximately 23 per cent, and if they share an entire X-chromosome, then it would be approximately 31 per cent. Therefore, MGMs and grandchildren are likely to share 25 per cent of their genomes, while PGM and granddaughter may share a total of approximately 31 per cent of their genes, with a likelihood of 27 per cent inheritance, while a PGM and grandson may share only approximately 23 per cent.

and their findings:

i’m posting this now as background because in future posts i’m going to bore you to tears looking at the differential inheritance of x- and y-chromosomes in families and cousin marriage. don’t say i didn’t warn you! (~_^)

previously: all cousins are not created equal

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