Final answer:
The primary cause of inbreeding depression is the exposure of deleterious recessive alleles as related individuals mate, leading to increased genetic disorders and reduced population fitness.
Step-by-step explanation:
The primary cause of inbreeding depression is the exposure of deleterious recessive alleles to purifying selection. Inbreeding refers to the mating of closely related individuals, which can bring together these harmful recessive mutations that would otherwise remain hidden within a population. This union can result in an increased incidence of genetic issues, leading to diminished fitness, abnormalities, and an increased susceptibility to diseases in the population.
Normally in outbred populations, rare deleterious alleles are less likely to pair up because of the greater genetic diversity. However, when inbreeding occurs, for example through cousin or sibling matings, the chance that two carriers of the same recessive allele will mate increases dramatically. Consequently, their offspring are more likely to inherit two copies of the recessive allele, manifesting the deleterious traits that lead to inbreeding depression. This process reduces the overall genetic health and adaptability of the population.