inbreeding

Relatives share ancestors, and therefore share alleles. When relatives mate, their offspring are more likely to receive two copies of the same ancestral allele—one down each side of the family. Inbreeding is mating between genetically related individuals, and its central genetic consequence is an increase in homozygosity.

Formally, inbreeding raises the chance that the two alleles an individual carries at a locus are identical by descent—copies of a single allele from a shared ancestor. This shifts genotype frequencies toward homozygotes and away from heterozygotes, even when allele frequencies themselves do not change. The degree is summarized by the inbreeding coefficient.

Because many harmful alleles are recessive and hidden in heterozygous carriers, more homozygosity means such alleles are more often expressed. The resulting drop in survival or fertility is called inbreeding depression, and it is a serious concern for small, isolated, or captive populations.

Inbreeding is not inherently 'bad,' though. Plant and animal breeders use controlled inbreeding to fix desirable traits and create uniform lines, and some species inbreed routinely. The risks rise specifically when rare deleterious recessive alleles are unmasked by homozygosity.