Inbreeding

Inbreeding is the production of offspring from the mating or breeding of individuals or organisms that are closely related genetically.^[2] By analogy, the term is used in human reproduction, but more commonly refers to the genetic disorders and other consequences that may arise from expression of deleterious recessive traits resulting from incestuous sexual relationships and consanguinity. Animals avoid incest only rarely.^[3]

Inbreeding results in

Inbreeding can significantly influence gene expression which can prevent inbreeding depression.^[14]

Overview

Offspring of biologically related persons are subject to the possible effects of inbreeding, such as

There may also be other deleterious effects besides those caused by recessive diseases. Thus, similar immune systems may be more vulnerable to infectious diseases (see Major histocompatibility complex and sexual selection).^[20]

Inbreeding history of the population should also be considered when discussing the variation in the severity of inbreeding depression between and within species. With persistent inbreeding, there is evidence that shows that inbreeding depression becomes less severe. This is associated with the unmasking and elimination of severely deleterious recessive alleles. However, inbreeding depression is not a temporary phenomenon because this elimination of deleterious recessive alleles will never be complete. Eliminating slightly deleterious mutations through inbreeding under moderate selection is not as effective. Fixation of alleles most likely occurs through Muller's ratchet, when an asexual population's genome accumulates deleterious mutations that are irreversible.^[21]

Despite all its disadvantages, inbreeding can also have a variety of advantages, such as ensuring a child produced from the mating contains, and will pass on, a higher percentage of its mother/father's genetics, reducing the recombination load,^[22] and allowing the expression of recessive advantageous phenotypes. Some species with a Haplodiploidy mating system depend on the ability to produce sons to mate with as a means of ensuring a mate can be found if no other male is available. It has been proposed that under circumstances when the advantages of inbreeding outweigh the disadvantages, preferential breeding within small groups could be promoted, potentially leading to speciation.^[23]

Genetic disorders

Children of parent-child or sibling-sibling unions are at an increased risk compared to cousin-cousin unions.[26]^: 3 Inbreeding may result in a greater than expected phenotypic expression of deleterious recessive alleles within a population.^[27] As a result, first-generation inbred individuals are more likely to show physical and health defects,^[28][29] including:

The isolation of a small population for a period of time can lead to inbreeding within that population, resulting in increased genetic relatedness between breeding individuals. Inbreeding depression can also occur in a large population if individuals tend to mate with their relatives, instead of mating randomly.

Due to higher

Island species are often very inbred, as their isolation from the larger group on a mainland allows natural selection to work on 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 permits the 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.^[34]

The reduced genetic diversity, for example due to a bottleneck will unavoidably increase inbreeding for the entire population. This may mean that 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. When 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 from the population those individuals who cannot survive because of illness.^{[citation needed]}

There was an assumption^{[by whom?]} 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 sexes, thought to be a mechanism by which the species instinctively avoids some of the genetic consequences of inbreeding.^[35] In general, many mammal species, including humanity's closest primate relatives, avoid close inbreeding possibly due to the deleterious effects.^[26]: 6

Examples

Although there are several examples of inbred populations of wild animals, the negative consequences of this inbreeding are poorly documented.^{[citation needed]} 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 was responsible for most of the individuals within the population. Even so, the diversity within the lines allowed great variation in the gene pool that may help to protect the South American sea lion from extinction.^[36]

In lions,

Cheetahs are another example of inbreeding. Thousands of years ago, the cheetah went through a population bottleneck that reduced its population dramatically so the animals that are alive today are all related to one another. A consequence from inbreeding for this species has been high juvenile mortality, low fecundity, and poor breeding success.[41]

In a study on an island population of song sparrows, individuals that were inbred showed significantly lower survival rates than outbred individuals during a severe winter weather related population crash. These studies show that inbreeding depression and ecological factors have an influence on survival.^[21]

The Florida panther population was reduced to about 30 animals, so inbreeding became a problem. Several females were imported from Texas and now the population is better off genetically.^[42][43]

Measures

A measure of inbreeding of an individual A is the probability F(A) that both alleles in one locus are derived from the same allele in an ancestor. These two identical alleles that are both derived from a common ancestor are said to be

Another useful measure that describes the extent to which two individuals are related (say individuals A and B) is their coancestry coefficient f(A,B), which gives the probability that one randomly selected allele from A and another randomly selected allele from B are identical by descent.[45] This is also denoted as the kinship coefficient between A and B.^[46]

A particular case is the self-coancestry of individual A with itself, f(A,A), which is the probability that taking one random allele from A and then, independently and with replacement, another random allele also from A, both are identical by descent. Since they can be identical by descent by sampling the same allele or by sampling both alleles that happen to be identical by descent, we have f(A,A) = 1/2 + F(A)/2.^[47]

Both the inbreeding and the coancestry coefficients can be defined for specific individuals or as average population values. They can be computed from genealogies or estimated from the population size and its breeding properties, but all methods assume no selection and are limited to neutral alleles.

There are several methods to compute this percentage. The two main ways are the path method^[48][44] and the tabular method.^[49][50]

Typical coancestries between relatives are as follows:

• Father/daughter or mother/son → 25% (1⁄4)
• Brother/sister → 25% (1⁄4)
• Grandfather/granddaughter or grandmother/grandson → 12.5% (1⁄8)
• Half-brother/half-sister, Double cousins → 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)

Animals

Wild animals

• Banded mongoose females regularly mate with their fathers and brothers.^[51]
• Bed bugs: North Carolina State University found that bedbugs, in contrast to most other insects, tolerate incest and are able to genetically withstand the effects of inbreeding quite well.^[52]
• Common fruit fly females prefer to mate with their own brothers over unrelated males.^[1]
• Cottony cushion scales: 'It turns out that females in these hermaphrodite insects are not really fertilizing their eggs themselves, but instead are having this done by a parasitic tissue that infects them at birth,' says Laura Ross of Oxford University's Department of Zoology. 'It seems that this infectious tissue derives from left-over sperm from their father, who has found a sneaky way of having more children by mating with his daughters.'^[53][54]
• eat them alive from the inside.^[55]

Domestic animals

Breeding in domestic animals is primarily assortative breeding (see selective breeding). Without the sorting of individuals by trait, a breed could not be established, nor could poor genetic material be removed.

Homozygosity is the case where similar or identical alleles combine to express a trait that is not otherwise expressed (recessiveness). Inbreeding exposes recessive alleles through increasing homozygosity.^[59]

Breeders must avoid breeding from individuals that demonstrate either homozygosity or heterozygosity for disease causing alleles.[60] The goal of preventing the transfer of deleterious alleles may be achieved by reproductive isolation, sterilization, or, in the extreme case, culling. Culling is not strictly necessary if genetics are the only issue in hand. Small animals such as cats and dogs may be sterilized, but in the case of large agricultural animals, such as cattle, culling is usually the only economic option.

The issue of casual breeders who inbreed irresponsibly is discussed in the following quotation on cattle:

Meanwhile, milk production per cow per lactation increased from 17,444 lbs to 25,013 lbs from 1978 to 1998 for the Holstein breed. Mean breeding values for milk of Holstein cows increased by 4,829 lbs during this period.^[61] High producing cows are increasingly difficult to breed and are subject to higher health costs than cows of lower genetic merit for production (Cassell, 2001).

Intensive selection for higher yield has increased relationships among animals within breed and increased the rate of casual inbreeding.

Many of the traits that affect profitability in crosses of modern dairy breeds have not been studied in designed experiments. Indeed, all crossbreeding research involving North American breeds and strains is very dated (McAllister, 2001) if it exists at all.^[62]

The BBC produced two documentaries on dog inbreeding titled Pedigree Dogs Exposed and Pedigree Dogs Exposed: Three Years On that document the negative health consequences of excessive inbreeding.

Linebreeding

Linebreeding is a form of inbreeding. There is no clear distinction between the two terms, but linebreeding may encompass crosses between individuals and their descendants or two cousins.^[58][63] This method can be used to increase a particular animal's contribution to the population.^[58] While linebreeding is less likely to cause problems in the first generation than does inbreeding, over time, linebreeding can reduce the genetic diversity of a population and cause problems related to a too-small gene pool that may include an increased prevalence of genetic disorders and inbreeding depression.^{[citation needed]}

Outcrossing

Outcrossing is where two unrelated individuals are crossed to produce progeny. In outcrossing, unless there is verifiable genetic information, one may find that all individuals are distantly related to an ancient progenitor. If the trait carries throughout a population, all individuals can have this trait. This is called the founder effect. In the well established breeds, that are commonly bred, a large gene pool is present. For example, in 2004, over 18,000 Persian cats were registered.^[64] A possibility exists for a complete outcross, if no barriers exist between the individuals to breed. However, it is not always the case, and a form of distant linebreeding occurs. Again it is up to the assortative breeder to know what sort of traits, both positive and negative, exist within the diversity of one breeding. This diversity of genetic expression, within even close relatives, increases the variability and diversity of viable stock.

Laboratory animals

Systematic inbreeding and maintenance of inbred strains of

animal model for experimental purposes and enables genetic studies in congenic and knock-out animals. In order to achieve a mouse strain that is considered inbred, a minimum of 20 sequential generations of sibling matings must occur. With each successive generation of breeding, homozygosity in the entire genome increases, eliminating heterozygous loci. With 20 generations of sibling matings, homozygosity is occurring at roughly 98.7% of all loci in the genome, allowing for these offspring to serve as animal models for genetic studies.^[65]

The use of inbred strains is also important for genetic studies in animal models, for example to distinguish genetic from environmental effects. The mice that are inbred typically show considerably lower survival rates.

Humans

See also: Incest, Incest taboo, Pedigree collapse, and Cousin marriage

Effects

Inbreeding increases homozygosity, which can increase the chances of the expression of deleterious or beneficial recessive alleles and therefore has the potential to either decrease or increase the fitness of the offspring. Depending on the rate of inbreeding, natural selection may still be able to eliminate deleterious alleles. ^[67]With continuous inbreeding, genetic variation is lost and homozygosity is increased, enabling the expression of recessive deleterious alleles in homozygotes. The coefficient of inbreeding, or the degree of inbreeding in an individual, is an estimate of the percent of homozygous alleles in the overall genome.^[68] The more biologically related the parents are, the greater the coefficient of inbreeding, since their genomes have many similarities already. This overall homozygosity becomes an issue when there are deleterious recessive alleles in the gene pool of the family.^[69] By pairing chromosomes of similar genomes, the chance for these recessive alleles to pair and become homozygous greatly increases, leading to offspring with autosomal recessive disorders.^[69] However, these deleterious effects are common for very close relatives but not for those related on the 3rd cousin or greater level, who exhibit increased fitness.^[3]

Inbreeding is especially problematic in small populations where the genetic variation is already limited.^[70] By inbreeding, individuals are further decreasing genetic variation by increasing homozygosity in the genomes of their offspring.^[71] Thus, the likelihood of deleterious recessive alleles to pair is significantly higher in a small inbreeding population than in a larger inbreeding population.^[70]

The fitness consequences of consanguineous mating have been studied since their scientific recognition by Charles Darwin in 1839.^[72][73] Some of the most harmful effects known from such breeding includes its effects on the mortality rate as well as on the general health of the offspring.^[74] Since the 1960s, there have been many studies to support such debilitating effects on the human organism.^{[71][72][74][75][76]} Specifically, inbreeding has been found to decrease fertility as a direct result of increasing homozygosity of deleterious recessive alleles.^[76][77] Fetuses produced by inbreeding also face a greater risk of spontaneous abortions due to inherent complications in development.^[78] Among mothers who experience stillbirths and early infant deaths, those that are inbreeding have a significantly higher chance of reaching repeated results with future offspring.^[79] Additionally, consanguineous parents possess a high risk of premature birth and producing underweight and undersized infants.^[80] Viable inbred offspring are also likely to be inflicted with physical deformities and genetically inherited diseases.^[68] Studies have confirmed an increase in several genetic disorders due to inbreeding such as blindness, hearing loss, neonatal diabetes, limb malformations, disorders of sex development, schizophrenia and several others.^[68][81] Moreover, there is an increased risk for congenital heart disease depending on the inbreeding coefficient (See coefficient of inbreeding) of the offspring, with significant risk accompanied by an F =.125 or higher.^[28]

Prevalence

The general negative outlook and eschewal of inbreeding that is prevalent in the Western world today has roots from over 2000 years ago. Specifically, written documents such as the Bible illustrate that there have been laws and social customs that have called for the abstention from inbreeding. Along with cultural taboos, parental education and awareness of inbreeding consequences have played large roles in minimizing inbreeding frequencies in areas like Europe. That being so, there are less urbanized and less populated regions across the world that have shown continuity in the practice of inbreeding.

The continuity of inbreeding is often either by choice or unavoidably due to the limitations of the geographical area. When by choice, the rate of consanguinity is highly dependent on religion and culture.

Hasidic and Haredi Jewish

groups.

Of the practicing regions, Middle Eastern and northern Africa territories show the greatest frequencies of consanguinity.[70]

Among these populations with high levels of inbreeding, researchers have found several disorders prevalent among inbred offspring. In

cleft lip/palate cases.^[70] Historically, populations of Qatar have engaged in consanguineous relationships of all kinds, leading to high risk of inheriting genetic diseases. As of 2014, around 5% of the Qatari population suffered from hereditary hearing loss; most were descendants of a consanguineous relationship.^[83]

Royalty and nobility

Main article: Royal intermarriage

See also: List of coupled cousins

extremely pronounced lower jaw

Inter-nobility

political alliances among elites. These ties were often sealed only upon the birth of progeny within the arranged marriage

. Thus marriage was seen as a union of lines of nobility and not as a contract between individuals.

Habsburg was known for its intermarriages; the Habsburg lip often cited as an ill-effect. The closely related houses of Habsburg, Bourbon, Braganza and Wittelsbach also frequently engaged in first-cousin unions as well as the occasional double-cousin

and uncle–niece marriages.

In

Ptolemy XIII, who married and became co-rulers of ancient Egypt following their father's death, are the most widely known example.^[87]

See also

• Álvarez incest case
• Coefficient of relationship
• Consanguinity
• Cousin marriage
• Cousin marriage in the Middle East
• Evolution of sexual reproduction
• Exogamy
• Founder effect
• F-statistics
• Fritzl case
• Genetic diversity
• Genetic purging
• Genetic sexual attraction
• Heterozygote advantage
• Identical ancestors point
• Inbreeding depression
• Inbreeding in fish
• Incest
• Incest taboo
• Insular dwarfism
• Intellectual inbreeding
• Legality of incest
• List of coupled cousins
• Mahram
• Outbreeding depression
• Outcrossing
• Proximity of blood
• Prohibited degree of kinship
• Selective breeding
• Self-incompatibility in plants
  (how some plants avoid inbreeding)

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External links

• Dale Vogt, Helen A. Swartz and John Massey, 1993. Inbreeding: Its Meaning, Uses and Effects on Farm Animals. University of Missouri, Extension. Archived 2012-03-08 at the Wayback Machine
• Consanguineous marriages with global map
• Ingersoll E (1920). "Cross-Fertilization in Animals and in Man" . Encyclopedia Americana.