Lecture 27
Effects of Inbreeding:
Today we will examine how inbreeding between close relatives (also known as
consanguineous matings) influences the appearance of autosomal recessive traits.
Note that inbreeding will not make a difference for dominant traits because they need
only be inherited from one parent or for X-linked traits since they are inherited from
the mother.
Consider an extreme case of inbreeding namely a brother-sister mating.
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A useful concept is the
Inbreeding Coefficient = F which is defined as the likelihood of
homozygosity by descent at a given locus.
If we consider a locus with different alleles in each grandparent: A1, A2, A3, A4,
F is the probability that the grandchild will be either A1/A1, A2/A2, A3/A3, A4/A4
p(A1/A1) = 1/2 . 1/2 . 1/4 = 1/16
"
p(A2/A2) = = 1/16
"
p(A3/A3) = = 1/16
"
p(A4/A4) = = 1/16
p(homozygous by descent) = 4 . 1/16 F = 1/4
A bother-sister mating is the simplest case but is of little practical consequence in human
population genetics since all cultures have strong taboos against this type of
consanguineous mating and the frequency is extremely low.
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