Inbreeding is breeding between close relatives.

If practised repeatedly, it typically leads to a reduction in genetic diversity. Inbreeding often leads to reduced health and fitness (called consanguinity depression); however, livestock breeders often practice inbreeding, then cull unfit offspring, especially when they are trying to establish a new and desirable trait in their stock.

Within humans, the supposed genetic problems caused by inbreeding are often cited as a factor rationalizing the taboos prohibiting incest.

The primary problem with inbreeding is that two closely related individuals are likely to have very similar genomes, and if one individual has a gene for a given negative trait, then the other is likely to have it as well. However, Gregor Mendel and his plant breeding experiments show that a single instance of inbreeding is statistically very unlikely to result in a flawed individual.

The most discussed instances of interbreeding relate to European monarchies. The royal families of Europe have close blood ties which are strengthened by intermarriage. Some examples from the British monarchy are:

• Queen Elizabeth II and Prince Philip are third cousins - both being descended from Queen Victoria, Philip is the great-great-grandson, Elizabeth II the great-great-granddaughter.

• Queen Victoria married her first cousin, Prince Albert.

• The House of Habsburg was famously weakened by intermarriage, leading to a number of birth defects.

However, it is not necessarily the case that there is a greater amount of inbreeding within royalty than there is in the population as a whole: it is simply better documented. Among genetic populations that are isolated, opportunities for exogamy are reduced. Isolation may be geographical, leading to inbreeding among peasants in remote mountain valleys. Or isolation may be social, induced by the lack of appropriate partners, such as Protestant princesses for Protestant royal heirs. Since the late middle ages, it is the urban middle class that has had the widest opportunity for outbreeding.

Inbreeding often occurs in animals. For example, the cheetah is a highly inbred species, probably because of a population bottleneck in the species' recent past. Inbreeding is also deliberately induced in laboratory mice in order to guarantee a consistent and uniform animal model. Human genetic diversity is also limited, indicating a population bottleneck some 70,000 years ago.

Purebred animals are often inbred; some critics argue the practice is unhealthy. [1] (http://www.dogbreedinfo.com/inbreeding.htm)

Where a species is threatened by extinction, the population may fall below a minimum whereby the forced interbreeding between the remaining animals will result in extinction.

See also

• Pedigree collapse
• Compare with: hybrid vigor
• Self-incompatibility in plants (how some plants avoid inbreeding)