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Inbreeding

Fitness effects

According to Icelandic records - it has records of family trees and marriages, going back centuries - the couples with the most descendants were 3rd or 4th cousins. The least related had the least number of descendants.

Coefficient of consanguinity

The coefficient of consanguinity (Cc or F) is the probability that the two allele genes that an individual has at a locus are identical by descendance. An indicator of how genetically similar your parents and ancestors are.

The probability that a child of half first cousins is homozygous by descent at a locus is F = $(½)^5$ = 1/32. In general, for autosomal loci, the inbreeding coefficient for an individual is F = $(½)^(n1+n2+1)$, where n1 and n2 are the numbers of generations separating the individuals in the consanguineous mating from their common ancestor. (This formula assumes that the common ancestor is not inbred.)

Eg. 0.054 is second cousin range.

Type of Mating	Proportion of Shared Alleles	F
Parent–offspring	1/2	1/4
Brother–sister	1/2	1/4
Half sibs	1/4	1/8
Uncle–niece, aunt–nephew	1/4	1/8
First cousins	1/8	1/16
Double first cousins	1/4	1/8
Half first cousins	1/16	1/32
First cousins once removed	1/16	1/32
Second cousins	1/32	1/64
Second cousins once removed	1/64	1/128
Third cousins	1/128	1/256