Loss of
heterozygosity due to inbreeding
Inbreeding in the
population genetic sense occurs when related individuals
mate with each other at higher than expected proportions.
Because all populations are finite, all individuals are
related to a greater or lesser extent: in smaller
populations, inbreeding necessarily occurs at a higher rate
than in larger.
Particular mating systems may
also produce extreme inbreeding. In a population of self-pollinating
plants segregating for two alleles A1
& A2, homozygotes of
either type [left & right] will therefore produce only
homozygous offspring. Any self-pollinating heterozygous
plants [middle] produce offspring in the expected 1:2:1
ratio. The homozygous offspring of either type are absorbed
into the corresponding homozygous lines. The heterozygous
offspring remain available for self-pollination in the next
generation. Each successive generation losses one-half the
proportion of heterozygous, and eventually converges on a
population that comprises two homozygous lines. If f(A1)
= p and f(A2) = q, Hexp
= 2pq but Hobs = 0.0.
The example shown is Blue-Eyed
Mary (Collinsia verna), a facultative out-crosser
that switches to self-pollination when insect
pollinators are scarce.