Relatedness pedigree

Relatedness in Pedigrees

The coefficient of relatedness (r) is the fraction of alleles that two individual have in common.
    If two individuals are related, their offspring will be inbred.

Disentangling genetic vs popular meaning of 'inbreeding': In the population genetic sense, inbreeding is the fraction of alleles shared because of common ancestry. Inbreeding does not require preferential mating of relatives (the popular understanding), it occurs necessarily in any closed population with a finite number of individuals, such that all individuals eventually become related to a greater or lesser degree. Some natural populations and human societies are measurably 'inbred' to a greater degree than expected: several human cultures favor brother-sister marriages as a means of retaining dynastic power or wealth.

    Ex.: Relatedness between parent & offspring is 1/2:
                the child inherits one-half its alleles from each parent.
 
            Relatedness between full sibs is also 1/2:
                each child inherits one-half is alleles from each parent,
                half of these will be inherited by the sib as well,
                and full sibs share two parents:
                (1/2)(1/2) + (1/2)(1/2) = 1/2
 
            Relatedness between first cousins is 1/8:
                the relatedness between their full-sib parents is 1/2,
                each cousin is related to its parent by 1/2:
                (1/2)(1/2)(1/2) = 1/8

Relatedness (r) & Inbreeding (F) coefficients for relatives & consanguineous matings
Relationship Description Example r F of offspring
Parent / Offspring Transmission of 1 of 2 alleles per locus
## 2 & 4 1/2 1/2 = (2)(1/2)(1/4)
Full sibs offspring of same parents ## 3 & 4 1/2 1/4 = (4)(1/4)2
Half sibs offspring with one parent in common not shown 1/4 1/8 = (2)(1/4)2
1st cousins offspring of full sibs ## 7 & 8 1/8 1/16 = (4)(1/8)2
2nd cousins offspring of 1st cousins ## 12 & 13 1/16 1/64 = (4)(1/16)2


Text material © 2021 by Steven M. Carr