INBREEDING

Deuteronomy 27:22

Viewing the King James Version. Click to switch to 1611 King James Version of Deuteronomy 27:22.

Cursed be he that lieth with his sister, the daughter of his father, or the daughter of his mother. And all the people shall say, Amen.

Inbreeding is reproduction from the mating of parents who are closely related genetically.Inbreeding results in increased 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.

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.

Results

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

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 for 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, 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 effect.

Examples

The cheetah was once reduced by disease, habitat restriction, overhunting of prey, and competition from other predators to a very small number of individuals.All cheetahs now come from this very small gene pool. Should a virus appear to which none of the cheetahs has resistance, extinction is always a possibility. Currently, the threatening virus is feline infectious peritonitis, which has a disease rate in domestic cats from 1–5%; in the cheetah population it is ranging between 50% to 60%. The cheetah is also known, in spite of its small gene pool, for few genetic illnesses.

In the South American sea lion, there was concern that recent population crashes would reduce genetic diversity. Historical analysis indicated that a population expansion from just two matrilineal lines were responsible for most individuals within the population. Even so, the diversity within the lines allowed for great variation in the gene pool that may help to protect the South American sea lion from extinction.

In lions, prides are often followed by related males in bachelor groups. When the dominant male is killed or driven off by one of these bachelors, a father may be replaced with his son. There is no mechanism for preventing inbreeding or to ensure outcrossing. In the prides, most lionesses are related to one another. If there is more than one dominant male, the group of alpha males are usually related. Two lines are then being "line bred". Also, in some populations such as the Crater lions, it is known that a population bottleneck has occurred. Researchers found far greater genetic heterozygosity than expected.In fact, predators are known for low genetic variance, along with most of the top portion of the tropic levels of an ecosystem.Additionally, the alpha males of two neighboring prides can potentially be from the same litter; one brother may come to acquire leadership over another's pride, and subsequently mate with his 'nieces' or cousins. However, killing another male's cubs, upon the takeover, allows for the new selected gene complement of the incoming alpha male to prevail over the previous male. There are genetic assays being scheduled for lions to determine their genetic diversity. The preliminary studies show results inconsistent with the outcrossing paradigm based on individual environments of the studied groups.

Calculation

Main article: Coefficient of relationship

The inbreeding is computed as a percentage of chances for two alleles to be identical by descent. This percentage is called "inbreeding coefficient". There are several methods to compute this percentage, the two main ways are the path methodand the tabular method.

Typical inbreeding coefficient percentages are as follows, assuming no previous inbreeding between any parents:

• Father/daughter, mother/son or brother/sister → 25% (¹⁄₄)
• Grandfather/granddaughter or grandmother/grandson → 12.5% (¹⁄₈)
• Half-brother/half-sister → 12.5% (¹⁄₈)
• Uncle/niece or aunt/nephew → 12.5% (¹⁄₈)
• Great-grandfather/great-granddaughter or great-grandmother/great-grandson → 6.25% (¹⁄₁₆)
• Half-uncle/niece or half-aunt/nephew → 6.25% (¹⁄₁₆)
• First cousins → 6.25% (¹⁄₁₆)
• First cousins once removed or half-first cousins → 3.125% (¹⁄₃₂)
• Second cousins or first cousins twice removed → 1.5625% (¹⁄₆₄)
• Second cousins once removed or half-second cousins → 0.78125% (¹⁄₁₂₈)
• Third cousins or second cousins twice removed → 0.390625% (¹⁄₂₅₆)
• Third cousins once removed or half-third cousins → 0.1953125% (¹⁄₅₁₂)

An inbreeding calculation may be used to determine the general genetic distance among relatives by multiplying by two, because any progeny would have a 1 in 2 risk of actually inheriting the identical alleles from both parents.

For instance, the parent/child or sibling/sibling relationships have 50% identical genetics