Inbreeding
[1] By analogy, the term is used in human reproduction, but more commonly refers to the genetic disorders and other consequences that may arise from expression of deleterious recessive traits resulting from incestuous sexual relationships and consanguinity.
[3] In extreme cases, this usually leads to at least temporarily decreased biological fitness of a population[4][5] (called inbreeding depression), which is its ability to survive and reproduce.
In the short term, incestuous reproduction is expected to increase the number of spontaneous abortions of zygotes, perinatal deaths, and postnatal offspring with birth defects.
[15] The advantages of inbreeding may be the result of a tendency to preserve the structures of alleles interacting at different loci that have been adapted together by a common selective history.
[16] Malformations or harmful traits can stay within a population due to a high homozygosity rate, and this will cause a population to become fixed for certain traits, like having too many bones in an area, like the vertebral column of wolves on Isle Royale or having cranial abnormalities, such as in Northern elephant seals, where their cranial bone length in the lower mandibular tooth row has changed.
Fixation of alleles most likely occurs through Muller's ratchet, when an asexual population's genome accumulates deleterious mutations that are irreversible.
It has been proposed that under circumstances when the advantages of inbreeding outweigh the disadvantages, preferential breeding within small groups could be promoted, potentially leading to speciation.
[31] Over time, with isolation, such as a population bottleneck caused by purposeful (assortative) breeding or natural environmental factors, the deleterious inherited traits are culled.
[6][7][32] Island species are often very inbred, as their isolation from the larger group on a mainland allows natural selection to work on their population.
[34] In general, many mammal species, including humanity's closest primate relatives, avoid close inbreeding possibly due to the deleterious effects.
[citation needed] In the South American sea lion, there was concern that recent population crashes would reduce genetic diversity.
Even so, the diversity within the lines allowed great variation in the gene pool that may help to protect the South American sea lion from extinction.
[36] In Central California, sea otters were thought to have been driven to extinction due to over hunting, until a small colony was discovered in the Point Sur region in the 1930s.
Population growth is limited by the fact that all Californian sea otters are descended from the isolated colony, resulting in inbreeding.
They can be computed from genealogies or estimated from the population size and its breeding properties, but all methods assume no selection and are limited to neutral alleles.
[60] The goal of preventing the transfer of deleterious alleles may be achieved by reproductive isolation, sterilization, or, in the extreme case, culling.
[63] However, US genomic technology has resulted in the US dairy cattle population becoming "the most inbred it’s ever been" and the rate of increase in US national milk yield has tapered off.
Systematic inbreeding and maintenance of inbred strains of laboratory mice and rats is of great importance for biomedical research.
In order to achieve a mouse strain that is considered inbred, a minimum of 20 sequential generations of sibling matings must occur.
With 20 generations of sibling matings, homozygosity is occurring at roughly 98.7% of all loci in the genome, allowing for these offspring to serve as animal models for genetic studies.
[68] With continuous inbreeding, genetic variation is lost and homozygosity is increased, enabling the expression of recessive deleterious alleles in homozygotes.
[72][73][75][76][77] Specifically, inbreeding has been found to decrease fertility as a direct result of increasing homozygosity of deleterious recessive alleles.
[77][78] Fetuses produced by inbreeding also face a greater risk of spontaneous abortions due to inherent complications in development.
[79] Among mothers who experience stillbirths and early infant deaths, those that are inbreeding have a significantly higher chance of reaching repeated results with future offspring.
[80] Additionally, consanguineous parents possess a high risk of premature birth and producing underweight and undersized infants.
[27] The general negative outlook and eschewal of inbreeding that is prevalent in the Western world today has roots from over 2000 years ago.
[71] Historically, populations of Qatar have engaged in consanguineous relationships of all kinds, leading to high risk of inheriting genetic diseases.
[84] In 2017-2019, congenital anomalies due to inbreeding was the most common cause of death of babies belonging to the Pakistani and Bangladeshi ethnic groups in England and Wales.
The closely related houses of Habsburg, Bourbon, Braganza and Wittelsbach also frequently engaged in first-cousin unions as well as the occasional double-cousin and uncle–niece marriages.
In ancient Egypt, royal women were believed to carry the bloodlines and so it was advantageous for a pharaoh to marry his sister or half-sister;[87] in such cases a special combination between endogamy and polygamy is found.
