Inbreeding

Inbreeding is breeding between close relatives, whether plant or animal. If practiced repeatedly, it often leads to a reduction in genetic diversity, and the accumulation of negative recessive traits, resulting in inbreeding depression. This may result in inbred individuals exibiting reduced health and fitness and lower levels of fertility.

Livestock breeders often practice inbreeding to "fix" desirable characteristics within a population. However, they must then cull unfit offspring, especially when trying to establish the new and desirable trait in their stock.

Results of inbreeding

Inbreeding may result in a far higher representation of deleterious genes within a population than would normally be expected. As a result inbred individuals are likely to show physical and health defects, including:

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.

The reduced genetic diversity that results from inbreeding may mean a species may not be able to adapt to changes in environmental conditions. Where a species becomes endangered, the population may fall below a minimum whereby the forced interbreeding between the remaining animals will result in extinction.

It is possible for a population to become inbred without inhertiting deleterious traits. For instance the cheetah is a highly inbred species, resulting from a population bottleneck. Many island species are also highly inbred.

Natural means to avoid inbreeding

Most animals and higher plants 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) 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.

Inbreeding in domestic animals

Inbreeding is used by breeders of domestic animals to fix desirable genetic traits within a population. This is often called line breeding within the livestock industry For instance an animal with a desirable colour is bred back to siblings or parents, on the understanding they may carry the genes for the colour without expressing them. Breeders must then cull unfit individuals, and when possible outbreed to unrelated stock to ensure deleterious traits are not also inherited.

Purebred animals are often inbred; some critics argue the practice is unhealthy. Many dog breeds have genetic diseases associated with their breed from this practice. ^*

Inbreeding is also deliberately induced in laboratory mice in order to guarantee a consistent and uniform animal model for experimental purposes. The problems with inbreeding are demonstrated by the fact that only about one in twenty inbred lines are sucessfully established.

Inbreeding in humans

Royalty and nobility

The royal and noble families of Europe have close blood ties which are strengthened by royal 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 in the past very inbred. Several Habsburgs, Bourbons and Wittelsbachs married aunts, uncles, nieces and nephews. 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. They are also third cousins as great-great-grandchildren of Queen Victoria. Other examples include:

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 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.
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
Prince Nicola Pignatelli (1648-1730) and Princess Giovanna Pignatelli (1666-1723) were half great-granduncle and half great-grandniece, representing an peculiar alliance between two relatives. Nicola was a son of Giulio Pignatelli, Prince of Noia (1587-1658) through his third wife and Giovanna a great-great-granddaughter through his first marriage.

Intermarriage in European royal families is no longer a problem, due to modern science and our understanding of the negative consequences, as well as the growing tendency to marry commoners. Also, it is not necessarily the case that there was a greater amount of inbreeding within royalty than there is in the population as a whole: it may simply be more well 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.