Inbreeding
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Inbreeding is breeding between close relatives.
If practiced repeatedly, it typically leads to a reduction in genetic diversity. Inbreeding often leads to reduced health and fitness (called consanguinity depression, inbreeding 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.
An inbred individual is likely to possess several physical and health defects, in addition to higher incidence of inheriting a poor trait. They include:
- reduced fertility both in litter size and in sperm viability
- increased congenital disorders
- fluctuating facial asymmetry
- lower birth rate
- higher infant mortality
- slower growth rate
- smaller adult size
- loss of immune system function.
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Means to avoid inbreeding
Mammals, most other animals, and higher plants as well, have ways to minimize inbreeding. They can be mechanical or societal.
An example of mechanical means is the sweet cherry. It has an elaborate biochemical mechanism that precludes self-fertilization and combination of gametes of high genetic similarity. Fruit flies, on the other hand, have a sensing mechanism to do the same thing, and more genetic diversity than expected by random mating is observed even in a closed population.
The incest taboo in humans is a societal means to avoid inbreeding. Mating with close relatives is often forbidden, although the definition of "close relatives" varies - it can include immediate family (parents, siblings), extended family (cousins) or even exclude whole generations (anyone of your father/mother's generation). Many pack or herd animals (such as lions, horses and dogs) inadvertently practice a social method to reduce inbreeding: young males are expelled from the group before they reach sexual maturity and might become competition for the alpha male, the only one to have sexual rights within his group.
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 perhaps some 70,000 years ago.
Purebred animals are often inbred; some critics argue the practice is unhealthy. [1]
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.
Inbred humans
Royalty
The royal families of Europe have close blood ties which are strengthened by intermarriage; the most discussed instances of interbreeding relate to European monarchies. Examples abound in every royal family; in particular, the ruling dynasties of Spain and Portugal were very inbred. Even in the British royal family, which is very moderate in comparison, there has scarcely been a monarch in 300 years who has not married a (near or distant) relative. Indeed, Queen Elizabeth II and her husband Prince Philip are second cousins once removed, both being descended from Christian IX of Denmark. They are also third cousins as great-great-grandchildren of Queen Victoria. Other examples include:
- Mary, Queen of Scots and Henry Stuart, Lord Darnley were half first cousins, and 3rd cousins once removed
- King William III and Queen Mary II were first cousins
- King George I and Princess Sophia Dorothea of Celle were paternal first cousins
- King George IV and Princess Caroline of Brunswick were first cousins
- Queen Victoria and Prince Albert of Saxe-Coburg-Gotha were first cousins
- King George V and Princess Mary of Teck were second cousins once removed
- The House of Habsburg intermarried very often. Famous in this case is the "Habsburger (Unter)Lippe" (Habsburg jaw/Habsburg lip/Austrian lip), typical for many Habsburg relatives over a period of six centuries, see Mandibular prognathism.
- Many Egyptian Pharaohs and Peruvian Incas married their sisters; in both cases we find a special combination between endogamy and polygamy. Normally the son of the old ruler and the ruler's oldest (half-)sister became the new one.
The last example also shows us, that inbreeding may not always tend to problems of health. Historical sources from the Spanish Conquistadores tell us, that the last Inca Atahualpa was an impressive looking man with a very high level of intelligence. Also his (half-)brother Huáscar isn't known to be unskilled. 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.
The Rothschilds
Among the descendants of Mayer Amschel Rothschild, the founder of the famous financial and banking family, many of the men married their brothers' daughters or cousins related through the male line, neither of which practices is forbidden by Jewish law. They also had the tradition that only male descendants in the male line could participate in the business, though daughters did inherit considerable wealth. These two traditions were a means of keeping the business closely in the family. This was the reason that, in 1901, the Frankfurt branch of the family business was closed when the male line that managed it died out.
See also
- Endogamy
- Exogamy
- Intermarriage
- Pedigree collapse
- Compare with: heterosis
- Self-incompatibility in plants (how some plants avoid inbreeding)
- F-statisticsda:Indavl