How inbreeding depression worksEdit
Inbreeding usually results from the exposure to deletrious recessive alleles to selection. Because these alleles are often recessive, a single wild type allele can still produce enough functional protein, in most instances, to produce a normal phenotype. However even tough they have no fitness consequences at all in the herterozygotes, loss-of-function mutation can be letahl in homozygotes. Thus, by increasing the proportion of individuals in a population that are homozygotes, inbreeding inccreases the frequency with which deletrious recessive affect phenotypes. Inbreeding depressions refers to the effect these alleles have on the average fitness of offspring in the population. Studies on humans have shown that inbreeding does, in fact, expose deletrious recessive alleles and consistently show that children of first cousins have higer mortality rates than children of unrelated parents (Bittles and Neel, 1994).
Bittles, A.H & Neel, J.V. (1994). The cost of human inbreeding and their implications for variations at the DNA level. Nature Genetics, 8, 117-121