Loss of heterozygosity due to inbreeding

    In common parlance, "inbreeding" means mating of close relatives, for example first-cousin marriages. 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: inbreeding necessarily occurs at a higher rate in small populations 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 [above, left] 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, H_exp = 2pq but H_obs = 0.0.

    The example shown is Blue-Eyed Mary (Collinsia verna), a facultative out-crosser that switches to obligate self-pollination when insect pollinators are scarce.