Inbreeding

Inbreeding is the mating of organisms that are closely related or that share a common ancestry. It is used deliberately by people to try to retain desirable traits and eliminate undesirable ones in animals. However, inbreeding can result in harmful recessive genes that had been masked in parents but later appear in the offspring.

Inbreeding of animals has been conducted by people ever since they first began to keep animals for food, clothing, and transport. It probably was done initially when a particularly useful characteristic was displayed in an animal, and the animal was then encouraged to mate with another of its kind that had the same, desirable trait. Today, inbreeding is used in animal husbandry, which is the scientific control and management of animals. It is performed for the same original purpose of encouraging the development of certain desirable traits in offspring. A good example of inbreeding is done with dogs and cats who are bred primarily for their appearance. There are also some types of inbreeding that occur in nature. Self-fertilization is one example that occurs in bisexual flowering plants. This is probably the most "inbred" an organism can be since its offspring are the result of the fusion of the male and female sex cells of the same individual. This particular form of inbreeding is sometimes necessary since it allows an isolated individual plant to create a local population. One disadvantage of such an inbred population, however, is that its ability to adapt to environmental changes is limited since its members all share the same pool of genes.

This limitation of inbreeding applies to animals as well, and all farmers know that they can only mate animal siblings (brothers and sisters) for a few generations before they start to show signs of being less healthy and less fertile. The reason for this is because of the same lack of variability (genetic differences) that the isolated flower population suffered. Inbreeding can cause harmful genes that are recessive in both parents to become expressed in their offspring. A recessive gene is not expressed if there is a dominant one to offset it or mask it. However, it is still part of the individual's genetic makeup. The recessive gene will be expressed if the other parent also has a recessive gene for the same trait.

Continued inbreeding can result in an accumulation of recessive genes, which can cause what is called "inbreeding depression." This is not a state of mind but rather a physical state that results in low fertility, poor general health, and particularly negative characteristics like stunted growth or feebleness. One example of a species that is threatened by its lack of genetic diversity is the cheetah. Because today's existing cheetahs are all so closely related, most have weakened immune systems and are very susceptible to disease. To avoid this, agricultural breeders sometimes practice a form of inbreeding called "linebreeding." This is accomplished by mating a female animal with its grandfather or uncle (rather than with a sibling or parent). This reduces the probability of undesirable genes in the offspring.

The opposite of inbreeding is outbreeding, which is defined as mating individuals that are not related at all. While this can make it more difficult to regularly achieve a certain desirable quality or trait, it does produce more vigorous and healthy offspring. The most extreme example of outbreeding is called crossbreeding in which individuals of different but closely related species are mated. The mule is an example of the cross-breeding of a horse and a donkey, and although it is a very strong and useful animal, the mule is nonetheless sterile or cannot reproduce. This is the case with all crossbred animals.

[See alsoGenetic Disorders ]