ok. back to business.

last time i regaled you with my new-and-improved coefficients of relationship/relatedness for the nuclear family members in all forms of cousin marriage (from the p.o.v. of a guy) — MBD (mother’s brother’s daughter) marriage, MZD (mother’s sister’s daughter) marriage, FBD (father’s brother’s daughter) marriage, and FZD (father’s sister’s daughter) marriage. (here’s a key to all the terms if you want to follow along. i also had neat diagrams of the four forms of cousin marriage in the previous post just in case, like me, you’re a visual sort-of person.).

now i’ve added the brothers & sisters as well as the grandparents. here we go (click on chart for LARGER image):

again, all of the family members (probably) share the most dna (i.e. alleles) in MZD marriage, and (ignoring the “no inbreeding at all” category) they share the least in FBD and FZD marriage. from what i’ve read, maternal cousin marriage is the most common globally (not in the middle east, tho), but i’m not sure which one — i.e. if it’s MBD or MZD. i’ll get back to you on that one. (see update below.)

to just repeat exactly what i said in the previous post (’cause i’m too lazy to write up something new and exciting):

the calculations are based on the fact that there is differential x- and y-chromosome inheritance from fathers and mothers to sons and daughters. this seems to matter in an inclusive fitness sort-of way between grandmothers and their grandchildren, so why not between all the other members of the family?

throw inbreeding into the mix, and you wind up with the members of some families being more related to one another than members of other families depending on the type of inbreeding (see chart).

here’s a couple of examples of how i did the math. please, tell me if you think my logic is wrong (very possible) and/or my calculations are wrong (very probable!).

first of all, the percentages of autosomal, x- and y-chromosomal dna that men and women have (based on the vega genome browser) are:

Female genome
6068 Mbp
Autosomal DNA: 5758 Mbp (≈94.89%)
X: 155 Mbp (≈2.55%), XX: 310Mb (≈5.11%)

Male genome
5972 Mbp
Autosomal DNA: 5758 Mbp (≈96.42%)
X: 155 Mbp (≈2.60%), Y: 59 Mbp (≈0.99%), XY: 214 Mbp (≈3.58%)

and here, the calculations for the genetic relatedness between paternal grandfather and son (PGF-s) when there is no inbreeding and in the four different types of cousin marriage:

PGF-s (no inbreeding)
**parents share no dna.
1/4 autosome + y-chromosome = (96.42% x 0.25) + 0.99% = 0.2510

PGF-s (MBD marriage)
**a man and his MBD share 1/8 of their autosomal dna + 1/4 x-chromosomal dna, so a PGF and a grandchild will share an additional 1/32 of their autosomal dna + 1/16 x-chromosomal dna.
1/4 autosome + 1/32 autosome + 1/16 x-chromosome + y-chromosome = (96.42% x 0.25) + (96.42% x 0.03125) + (2.60% x 0.0625) + 0.99% = 0.2827

PGF-s (MZD marriage)
**a man and his MZD share 1/8 of their autosomal dna + 1/4 x-chromosomal dna + 1/8 x-chromosomal dna, so a PGF and a grandchild will share an additional 1/32 of their autosomal dna + 1/16 x-chromosomal dna + 1/32 x-chromosomal dna.
1/4 autosome + 1/32 autosome + 1/16 x-chromosome + 1/32 x-chromosome + y-chromosome = (96.42% x 0.25) + (96.42% x 0.03125) + (2.60% x 0.0625) + (2.60% x 0.03125) + 0.99% = 0.2835

PGF-s (FBD marriage)
**a man and his FBD share 1/8 of their autosomal dna, so a PGF and a grandchild will share an additional 1/32 of their autosomal dna.
1/4 autosome + 1/32 autosome + y-chromosome = (96.42% x 0.25) + (96.42% x 0.03125) + 0.99% = 0.2811

PGF-s (FZD marriage)
**a man and his FZD share 1/8 of their autosomal dna, so a PGF and a grandchild will share an additional 1/32 of their autosomal dna.
1/4 autosome + 1/32 autosome + y-chromosome = (96.42% x 0.25) + (96.42% x 0.03125) + 0.99% = 0.2811

rinse and repeat for all the other family members.

update 07/04: apparently, MBD marriage (matrilateral cross cousin marriage) is the most common form of cousin marriage.

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