Inbreeding

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Definitions

  • 1: the interbreeding of closely related individuals especially to preserve and fix desirable characters of and to eliminate unfavorable characters from a stock
  • 2: confinement to a narrow range or a local or limited field of choice

Description

Inbreeding is reproduction from the mating of pairs who are closely related genetically. Inbreeding results in homozygosity, which can increase the chances of offspring being affected by recessive or deleterious traits. This generally leads to a decreased fitness of a population, which is called inbreeding depression. An individual who results from inbreeding is referred to as inbred. The avoidance of expression of deleterious recessive alleles caused by inbreeding is thought to be the main selective force maintaining the outcrossing aspect of sexual reproduction.

Livestock breeders often practice controlled breeding to eliminate undesirable characteristics within a population, which is also coupled with culling of what is considered unfit offspring, especially when trying to establish a new and desirable trait in the stock.

In plant breeding, inbred lines are used as stocks for the creation of hybrid lines to make use of the effects of heterosis. Inbreeding in plants also occurs naturally in the form of self-pollination.

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

  • Reduced fertility both in litter size and sperm viability
  • Increased genetic disorders
  • Fluctuating facial asymmetry
  • Lower birth rate
  • Higher infant mortality
  • Slower growth rate
  • Smaller adult size
  • Loss of immune system function

Inbreeding can occur just because a small population has been isolated during some time, so that all breeding individuals became genetically related. It can also occur in a large population if individuals tend to mate their relatives, instead of mating at random.

Many individuals in the first generation of inbreeding will never live to reproduce. Over time, with isolation such as a population bottleneck caused by purposeful (assortative) breeding or natural environmental factors, the deleterious inherited traits are culled.

Island species are often very inbred, as their isolation from the larger group on a mainland allows natural selection to work upon their population. This type of isolation may result in the formation of race or even speciation, as the inbreeding first removes many deleterious genes, and allows expression of genes that allow a population to adapt to an ecosystem. As the adaptation becomes more pronounced the new species or race radiates from its entrance into the new space, or dies out if it cannot adapt and, most importantly, reproduce.

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

Natural breedings include inbreeding by necessity, and most animals only migrate when necessary. In many cases, the closest available mate is a mother, sister, grandmother, father, brother, or grandfather. In all cases, the environment presents stresses to remove those individuals who cannot survive because of illness from the population.

There was an assumption that wild populations do not inbreed; this is not what is observed in some cases in the wild. However, in species such as horses, animals in wild or feral conditions often drive off the young of both genders, thought to be a mechanism by which the species instinctively avoids some of the genetic consequences of inbreeding. In general, many mammal species including humanity's closest primate relatives avoid close inbreeding possibly due to the deleterious effects.[1]