Overview

Inbreeding refers to reproduction between individuals that are more closely related than average members of a population. The term is used in genetics and applied to animals, plants and other sexually reproducing organisms. At one extreme of the spectrum is self-fertilization in plants, where a single individual provides both gametes; at the other extreme are matings between unrelated or distantly related individuals, often called outcrossing. In many cultivated plants both male and female flowers occur on the same plant and selfing is common; see also species with combined floral sexes.

Common forms and measurement

Common forms of inbreeding include parent–offspring mating, sibling mating and cousin mating; extreme cases yield inbred strains that show very low genetic variation. The genetic consequence of these matings is an increase in homozygosity: individuals are more likely to inherit identical copies of a gene from both parents. Geneticists quantify this tendency with an inbreeding coefficient (often denoted F), which measures the probability that two alleles at a locus are identical by descent.

Genetic effects and consequences

Elevated homozygosity can expose recessive deleterious alleles that are harmless in heterozygous state. The resulting loss of biological fitness—reduced fertility, slower growth, greater susceptibility to disease, or higher juvenile mortality—is commonly called inbreeding depression. In some controlled breeding programs, sustained inbreeding may allow selection to remove strongly deleterious alleles, a process called purging, but purging is incomplete and risky because it depends on the genetic architecture of fitness traits. In contrast, outcrossing can restore genetic diversity and produce heterosis (hybrid vigor).

Uses, management and examples

Inbreeding has practical uses as well as drawbacks. Plant and animal breeders use controlled inbreeding together with strategic outcrossing as tools for developing uniform lines or fixing desirable traits; laboratory animals are often maintained as nearly inbred strains for reproducible experiments. Conversely, conservation programs for endangered species must manage inbreeding to avoid loss of fitness and adaptive potential; techniques such as genetic rescue, captive breeding, and deliberate outcrossing are used when populations become too small. Examples include pedigree dog breeds that show breed-specific genetic disorders, historical royal family lineages noted for concentrated relatedness, and many self-pollinating crop plants.

In humans, close-kin reproduction raises medical, ethical and social concerns because of increased risk of recessive genetic conditions. Nearly all societies regulate or discourage close consanguineous unions, and many maintain explicit prohibitions or limits—for instance, restrictions on marriage between first cousins are common in some regions but vary substantially worldwide. Public health advice, genetic counseling and carrier screening are tools used to assess and manage the risks in communities where consanguineous unions occur.

Distinctions, history and further reading

Inbreeding should be distinguished from assortative mating (preference-based pairing) and from hybridization between divergent groups. The concept has long been recognized in agriculture and animal husbandry; after Mendelian genetics and later population genetic theory, its causes and consequences became clearer and measurable. For introductions and technical treatments consult sources on genetics, the biology of self-fertilizing plants (flower morphology), and discussions of outcrossing versus inbreeding. Practical breeding and conservation approaches are treated in literature on artificial selection and population management, and legal or cultural perspectives are discussed in works addressing kinship and marriage norms (species boundaries and social norms, cousin marriage regulations).