Inbreeding

Inbreeding is reproduction between individuals that share ancestry, especially when they are more closely related than typical members of a reference population. It increases the likelihood that offspring inherit copies of the same ancestral allele from both parents, thereby increasing homozygosity and reducing heterozygosity. Inbreeding occurs naturally and in managed breeding populations; its consequences depend on the degree of relatedness, the genetic variants present, and the population’s history. Reduced survival or reproductive success associated with inbreeding is called inbreeding depression. (nature.com)

Genetic basis

In a diploid organism, most genes occur in two copies, normally inherited one from each parent. An individual is homozygous at a locus when these copies carry the same allele and heterozygous when they carry different alleles. Related parents have an increased probability of transmitting alleles inherited from a common ancestor. Copies inherited in this way are described as identical by descent; homozygosity resulting from identity by descent is also called autozygosity. Alleles can be identical in their observed sequence without establishing this particular ancestral relationship. (fao.org)

Inbreeding primarily changes the distribution of genotypes, rather than directly changing allele frequencies. For a locus with alleles $A$ and $a$, with frequencies $p$ and $q$, where $p+q=1$, the standard inbreeding model gives:

Here $F$ measures inbreeding relative to a specified ancestral population. Compared with Hardy–Weinberg equilibrium, heterozygotes decrease and both homozygote classes increase. Allele frequencies can subsequently change through selection and other evolutionary processes. (fao.org)

This distinction separates inbreeding from genetic drift, which is random change in allele frequencies. Both processes are important in small populations, but they are not interchangeable. (fao.org)

Forms and circumstances

Inbreeding can result from several reproductive arrangements:

• Close-relative mating, including reproduction between siblings, parents and offspring, or cousins.
• Repeated breeding within a small, closed population, in which shared ancestry accumulates even when immediate relatives are not deliberately paired.
• Self-fertilization, in which one individual supplies both gametes. It is an especially strong form of inbreeding in organisms capable of it.
• Deliberate inbreeding, used to establish genetically uniform lines or concentrate ancestry from selected breeding animals. (fao.org)

In human population studies, consanguinity denotes biological relatedness, whereas endogamy denotes marriage within a community or population. They are not equivalent: community membership alone does not specify a couple’s degree of genetic relationship. (nature.com)

Measuring inbreeding

Pedigree coefficients

The inbreeding coefficient, usually written $F$, is the probability that the two alleles at an autosomal locus in an individual are identical by descent relative to the pedigree’s reference ancestors. It is not the probability that the individual will have a genetic disorder. Pedigree estimates depend on the recorded relationships and assumptions about the founders. (fao.org)

For simple pedigrees with unrelated, non-inbred founders, illustrative offspring coefficients are:

Additional shared ancestry can increase these values. Offspring with the same pedigree coefficient need not inherit exactly the same amount or distribution of autozygous DNA. (fao.org)

Genomic measurements

Genomic approaches detect runs of homozygosity (ROH): stretches of the genome that are homozygous across consecutive markers. A commonly used measure, $F_{\mathrm{ROH}}$, is the fraction of the assessed genome contained in these stretches. Long ROH generally indicate more recent shared ancestry, whereas shorter stretches can reflect more distant ancestry. Estimates depend on marker density and the criteria used to identify a run. (nature.com)

Pedigree and genomic measurements therefore capture related but distinct information: pedigrees describe expected inheritance from recorded ancestors, while genomic measurements reveal the segments actually inherited. (nature.com)

Inbreeding depression

Inbreeding depression is a reduction in biological fitness, especially survival and fertility, associated with increased inbreeding. It has been documented in wild plants and animals as well as managed populations. Its magnitude varies among populations, traits, and stages of life. (nature.com)

The principal genetic explanation is the increased expression of harmful recessive or partially recessive variants. Such variants can have little effect in heterozygotes but reduce fitness when present in two copies. Inbreeding makes these homozygous combinations more frequent. A second mechanism is the loss of heterozygote advantage at loci where heterozygotes have higher fitness than either homozygote. Evidence indicates that recessive deleterious variants account for much inbreeding depression, although distinguishing mechanisms can be difficult when neighboring genes are inherited together. (nature.com)

Inbreeding does not make every offspring unhealthy. Its coefficient describes an inheritance probability, not a predetermined phenotype. In a long-term study of wild Soay sheep, genomic inbreeding was associated with reduced survival, illustrating how effects can be assessed in natural populations rather than inferred from appearance alone. (nature.com)

Genetic purging

Increased expression of harmful recessive variants can expose them to natural selection, lowering their frequencies over generations. This process is called genetic purging. It does not guarantee that an inbred population becomes free of harmful variation. (pubmed.ncbi.nlm.nih.gov)

Research on small, isolated Indian tiger populations found evidence consistent with removal of some highly deleterious variants, while remaining harmful variants could still impose inbreeding-associated costs. Purging and continuing genetic vulnerability can therefore coexist. (pmc.ncbi.nlm.nih.gov)

Population size and conservation

In a finite, closed population, individuals increasingly share ancestors over generations. The rate of accumulation depends on effective population size, $N_e$, rather than simply the number of individuals counted. Unequal reproductive success, skewed sex ratios, and population contractions can make effective size substantially smaller than census size. (fao.org)

Under a standard idealized diploid model, the proportional increase in inbreeding per generation is approximately

Thus, small effective populations accumulate inbreeding faster. The expression describes a rate relative to remaining non-autozygous variation, not a constant absolute addition to $F$ regardless of its existing value. (fao.org)

In conservation biology, inbreeding is considered alongside loss of genetic diversity, habitat isolation, and demographic decline. Introducing ancestry from another population can increase fitness through genetic rescue, but outcomes depend on population differences and subsequent reproduction; it is not a universal remedy. (pubmed.ncbi.nlm.nih.gov)

A documented example is the Florida panther population. A genetic-rescue intervention in 1995 introduced eight female pumas from Texas. Analysis of nine generations found sustained improvements in genetic, morphological, and demographic measures associated with inbreeding depression. (pmc.ncbi.nlm.nih.gov)

Uses in breeding and research

In crop breeding, inbred lines provide relatively stable parental genotypes. Crossing different lines can produce heterosis, or hybrid vigor, including increased yield relative to the parental lines. This benefit does not mean that inbreeding itself improves vigor; the production of uniform parental lines and the performance of their crosses are distinct stages. (fao.org)

In livestock breeding and aquaculture, uncontrolled inbreeding can reduce productive performance. Heavy use of a small number of successful breeding animals can lower effective population size even when the total population is large. Breeding programs consequently measure ancestry and genetic contributions as well as desirable production traits. (fao.org)

Laboratory inbred strains provide reproducible genetic backgrounds for experiments. In mouse nomenclature, an inbred strain is established through at least 20 consecutive generations of sibling or parent–offspring mating, or equivalent descent from a single ancestral pair. Such strains are homozygous at virtually all loci, but they are not permanently immutable: new variants and separation into substrains can produce genetic differences. (jax.org)

Human genetics

Related biological parents have an increased probability of carrying the same inherited recessive disease-causing variant. For an autosomal recessive disorder, when both parents carry a disease-causing variant in the same gene, each pregnancy ordinarily has a one-in-four probability of producing an affected child. This conditional probability is not the overall disease risk for all children of related parents. (nature.com)

Human studies distinguish pedigree relatedness, genomic autozygosity, and social or environmental factors that may accompany particular marriage patterns. Observed associations require careful interpretation because these factors can be correlated. (nature.com)

Historical development

Charles Darwin investigated the consequences of self-fertilization and cross-fertilization through multigenerational plant experiments. His The Effects of Cross and Self Fertilisation in the Vegetable Kingdom, published in 1876, documented differences in growth and fertility and helped establish the biological importance of mating relationships before their molecular basis was understood. (darwin-online.org.uk)

Twentieth-century genetics connected these observations to recessive inheritance and genotype frequencies. Later genomic methods made it possible to measure inherited homozygous segments directly and examine the effects of inbreeding in populations without complete pedigrees. (nature.com)

References

1. Inbreedingnature.com
2. The genetics of inbreeding depressionnature.com
3. Runs of homozygosity: windows into population history and trait architecturenature.com
4. Inbreeding and brood stock management: Genetic principlesfao.org
5. Inbreeding and brood stock management: Inbreedingfao.org
6. Inbreeding and brood stock management: Effective breeding numberfao.org
7. Genetic architecture and lifetime dynamics of inbreeding depression in a wild mammalnature.com
8. Understanding Inbreeding Depression, Purging, and Genetic Rescuepubmed.ncbi.nlm.nih.gov
9. Genomic evidence for inbreeding depression and purging of deleterious genetic variation in Indian tigerspmc.ncbi.nlm.nih.gov
10. The State of the World’s Animal Genetic Resources for Food and Agriculturefao.org
11. FAO reportfao.org
12. Multi-generational benefits of genetic rescuepmc.ncbi.nlm.nih.gov