Inbreeding

Inbreeding is a term in genetics, meaning the crossing (mating) of closely related animals or plants. Self-fertilisation in plants is the most extreme kind of inbreeding. It is quite widespread in plants which carry both male and female flowers on the same plant. Inbreeding is the opposite of outcrossing, which is the mating of unrelated members of the same species. The combination of inbreeding and outcrossing is a standard method of artificial selection.

Other terms are inbred strain, a group of organisms so inbred that they have little or no genetic variation, and inbreeding depression, which is the decreased fitness (usually lack of fertility and early illness and death) brought about by inbreeding.

Inbred strains, for example of mice or drosophila, are often used in biology as model organisms when genetic uniformity makes them useful in investigation of such things as drugs and cancer.

For human beings, inbreeding is destructive, and almost all cultures have an incest taboo. For example, many prohibit marriage between first cousins, and most prohibit it for closer family members.

Loss of genetic variation

Loss of genetic variation means that many gene loci that were heterozygous are now homozygous. The result of both genes at a locus being mutant can be serious.

The most intense form of inbreeding is the self-fertilisation in an hermaphrodite, in which the proportion of heterozygotes is halved in each generation.^p139 Aa x Aa (100%) gives 1 AA: 2 Aa; 1 aa (50%), and so on. A similar effect takes twice as long with brother-sister mating, and longer still with mating in very small isolated groups. The latter situation is famous because Sewall Wright did his calculations for just such groups, and he called the process genetic drift. Genetic drift and inbreeding are closely related concepts.

The degree of inbreeding can be measured in various ways. A simple test for mammals is the exchange of skin grafts. If the grafts are not rejected, the animals must be nearly identical genetically. Skin grafts can be successfully exchanged between rats in standard laboratory strains, and between cheetahs caught in the wild.

Discussion

Why is it that inbreeding brings about a loss of viability? There are two answers, and both seem to be true. The first is that a group with little genetic variation is vulnerable to environmental challenges: infections, sudden climate events, predators. Small groups are vulnerable to chance events in any case, but with no heritable variability they are even more vulnerable.

The second explanation is genetic. Some of the homozygous genes will be deleterious recessives which would normally be shielded by a dominant allele. Also, there are some loci where the heterozygote is inherently fitter than either homozygote. If so, inbreeding will automatically cause a loss of vigour.^p103

Many plant species do self-pollination in the wild, but the great majority have occasional outcrossing. This provides them with sufficient variety for their survival.

Inbreeding Media

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  Common fruit fly females prefer to mate with their own brothers over unrelated males.

• Animation of uniparental isodisomy

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  Heterozygous refers to having inherited different forms of a particular gene from each parent. A heterozygous genotype stands in contrast to a homozygous genotype, where an individual inherits identical forms of a particular gene from each parent.

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  White tiger in Gunma Safari Park

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  Hereditary polycystic kidney disease is prevalent in the Persian cat breed, affecting almost half the population in some countries.

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  An intensive form of inbreeding where an individual S is mated to his daughter D1, granddaughter D2 and so on, in order to maximise the percentage of S's genes in the offspring. 87.5% of D3's genes would come from S, while D4 would receive 93.75% of their genes from S.

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  Global prevalence of consanguinity

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  Carlos II, a member of the famously inbred Habsburgs with an extremely pronounced lower jaw