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Inbreeding is a genetic term that refers to reproduction as a result of the mating of two animals which are genetically related to each other. If the relationship is a close one or it is practiced repeatedly, inbreeding 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. Deleterious alleles causing inbreeding depression can subsequently be removed through culling, which is also known as genetic purging.

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.

In plant breeding, inbred lines are used as stocks for the creation of hybrid lines to make use of the heterosis effect. 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:

Natural selection works to remove individuals who acquire the above types of traits from the gene pool. Therefore, many more 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 stress, the deleterious inherited traits are culled.

The cheetah once was reduced by disease, habitat restriction, overhunting of prey, competition from other predators (primarily lions, competition from human land use, etc.) to a very small number of individuals. All cheetahs now come from this very small gene pool. Should a virus appear that none of the cheetahs have resistance to, 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.

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.

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 inoculate the South American sea lion from extinction.

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

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 then are 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.

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.


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 method [8796] and the tabular method [8797].

Typical inbreeding percentages are as follows:
  • Father/daughter – mother/son – brother/sister → 25%
  • Half-brother/half-sister → 12.5%
  • Uncle/niece – aunt/nephew → 12.5%
  • Cousin → 6.25%

An inbreeding calculation may be used to determine the general genetic distance among relatives by multiplying by 2, 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 have 50% identical genetics.NOTE: For siblings, the degree of genetic relationship is not an automatic 50% (as it is with parents and their children), but a range from 100% at one extreme – as in the case of identical twins [who obviously could not mate as they are the same sex] – to an exceedingly unlikely 0%. Siblings share an average of 50% of their genes, but unlike the 50% ratio between parents and children, the actual ratio between siblings in any given case can vary widely.

Domestic animals

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

Inbreeding is used by breeders of domestic animals to fix desirable genetic traits within a population or to attempt to remove deleterious traits by allowing them to manifest phenotypically from the genotypes. Inbreeding is defined as the use of close relations for breeding such as mother to son, father to daughter, brother to sister. Breeders must cull unfit breeding suppressed individuals and/or individuals who demonstrate either homozygosity or heterozygosity for genetic based diseases. 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. 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.

Linebreeding, which is a milder form of inbreeding is accomplished through breeding of cousins, aunt to nephew, half brother to half sister. This was used to isolate breeds within the companion and livestock industry. For instance an animal with a desirable colour is bred back within the lines with identified selection traits whether it be milk production or adherence to breed standard of appearance or behavior. Breeders must then cull unfit individuals, and in some cases the breeders will then outbreed to increase the level of genetic diversity. Again casual breeding is problematic as it is without the requisite culling of individuals who are either maladaptive, not to breed standard or carriers of poor genetic material that must be removed from a healthy breeding program.

Outcrossing is where two unrelated individuals have been 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's 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. 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.

The two dog sites above also point out that in the registered dog population, the onset of large numbers of casual breeders has corresponded with an increase in the number of genetic illnesses of dogs by not understanding how, why and which traits are inherited. The dog sites indicate that the largest percentage of dog breeders in the US are casual breeders. Therefore the investment in a papered animal,with an expected short term profit, motivates some to ignore the practice of culling. Casual breeders in companion animals often ignore breeding restrictions within their contracts with source companion animal breeders. The casual breeders breed the very culls that a genetics based breeder has released as a pet. The casual breeder also was cited in the quotes above on cattle raising.

Inbreeding is also deliberately induced in laboratory mice in order to guarantee a consistent and uniform animal model for experimental purposes.


Genetic disorders

The offspring of consanguineous relationship are at greater risk of certain genetic disorders. These autosomal recessive disorders occur in individuals who are homozygous for a particular recessive gene mutation. This means that they carry two copies (alleles) of the same gene. Except in certain rare circumstances (new mutations or uniparental disomy) both parents of an individual with such a disorder will be carriers of the gene. Such carriers are not affected and will not display any signs that they are carriers, and so may be unaware that they carry the mutated gene. As relatives share a proportion of their genes, it is much more likely that related parents will be carriers of an autosomal recessive gene, and therefore their children are at a higher risk of an autosomal recessive disorder. The extent to which the risk increases depends on the degree of genetic relationship between the parents; so the risk is greater in mating relationships where the parents are close relatives, but for relationships between more distant relatives, such as second cousins, the risk is lower (although still greater than the general population).

Prohibitions to inbreeding

The taboo of incest has been discussed by many social scientists. Anthropologists attest that it exists in most cultures. As inbreeding within the first generation often produces expression of recessive traits, the prohibition has been discussed as a possible functional response to the requirement of culling those born deformed, or with undesirable traits. Some biologists like Charles Davenport advocated the traditional forms of assortative breeding, i.e. eugenics, to form better "human stock".

Ancient Egypt

In ancient Egypt, some Pharaohs married their sisters; in such cases a special combination between endogamy and polygamy is found. Normally the old ruler's eldest son and daughter (who could be either siblings or half-siblings) became the new rulers. All rulers of the Ptolemaic dynasty from Ptolemy II were married to their brothers and sisters, so as to keep the Ptolemaic blood "pure" and to strengthen the line of succession. Cleopatra VII and Ptolemy XIII, who married and became co-rulers of ancient Egypt following their father's death, are the most widely known example.

Royalty and nobility

In discussing humans, the term inbreeding is highly offensive and judgmental. Roughly 20% of the world's marriages are between first cousins, and such marriages are not illegal in most of the world. Although it is an undisputed fact that cousin marriages increase the probability of genetic disease, the level of statistical increase varies with the degree of relationship, and the frequency of the marriages. The casual use of the term inbred implies that some degree of degradation exists, when in fact there may be no effect at all. The family relationships of royalty are usually very well known leading observers to view royalty as highly inbred, but they are often comparable to many ethnic groups where the relationships are not publicized as well. The royal and noble families of Europe have traditionally been prone to royal intermarriage, as it protected property, wealth, and position.

European monarchies Jean V of Armagnac formed an extremely rare brother-sister marriage. In particular, the Habsburg up until the year 1700 had a great deal of intermarriage. The line died out leading to the War of the Spanish Succession. Also other royal houses, such as the Wittelsbachs had marriages among aunts, uncles, nieces, and nephews. The Britishmarker royal family had several marriages as close as the first cousin, but none closer.

The most famous example of a genetic disorder aggravated by royal family intermarriage was the House of Habsburg, which inmarried particularly 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. The condition progressed through the generations to the point that the last of the Spanish Habsburgs, Charles II of Spain, could not properly chew his food. (See mandibular prognathism.)

Besides the jaw deformity, Charles II also had a huge number of other genetic physical, intellectual, sexual, and emotional problems. It is speculated that the simultaneous occurrence in Charles II of two different genetic disorders: combined pituitary hormone deficiency and distal renal tubular acidosis could explain most of the complex clinical profile of this king, including his impotence/infertility which in last instance led to the extinction of the dynasty.

The most famous genetic disease that circulated among European royalty was hemophilia. Because the progenitor, Queen Victoria, was in a first cousin marriage, it is often mistakenly believed that the cause was consanguinity. However, this disease is not aggravated by cousin marriages.

Many more examples of royal couples are included in the article list of coupled cousins.

Intermarriage within European royal families has declined in relation to the past. Inter-nobility marriage was used as a method of forming political alliances among elite power-brokers. 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, not of a contract between individuals as it is seen today.

Isolated groups

Among genetic populations that are isolated, opportunities for exogamy are reduced, however may not intend to inbreed. Isolation may be geographical, leading to inbreeding among people in remote mountain valleys. Or isolation may be social, induced by the lack of appropriate partners, such as Protestant princesses for Protestant royal heirs, in which case inbreeding is desired. Since the late Middle Ages, it is the urban middle class that has had the widest opportunity for outbreeding and the least desire to inbreed.

  • Some Peruvian Sapa Incas married their sisters; in such cases a special combination between endogamy and polygamy is found. Normally the son of the old ruler and the ruler's oldest (half-)sister became the new ruler.
  • The Inca had an unwritten rule that the new ruler must be a son of the Inca and his wife and sister. He then had to marry his sister (not half-sister), which ultimately led to the catastrophic Huascars reign, culminating in a civil war and then fall of the empire.

Icelandic study

A recent study in Iceland by the deCODE genetics company, published by the journal Science, found that third cousins had the highest rate of genetic success and children, suggesting a minimal relationship to each other is favorable in humans pairing off and reproducing.

Allele exposure

Genes express themselves according to the way they pair with each other as alleles. One case of this is known as homozygosity. It is the case where similar or identical alleles combine to express a trait that is not otherwise expressed (recessiveness). In other words, inbreeding, through homozygosity, exposes recessive alleles. Culling is a case of a use of this.


Inbreeding has a variety of consequences. Allele exposure can cause genes to be expressed that are not otherwise expressed. This fact, combined with the fact that most mutations are recessive may indicate that inbreeding drives evolution. Speciation, a key process in evolution, depends on reproductive barriers, a necessary feature of which is inbreeding.

See also


  1. M Menotti-Raymond and S J O'Brien. "Dating the genetic bottleneck of the African cheetah." Proc Natl Acad Sci U S A. 1993 April 15; 90(8): 3172–3176.
  6. "ADVS 3910 Wild Horses Behavior," web page accessed June 22, 2007 at
  7. How to compute and inbreeding coefficient (the path method)
  8. G2036 Culling the Commercial Cow Herd: BIF Fact Sheet, MU Extension
  9. Homepage: S1008
  10. Inbreeding and linebreeding Retrieved on 2009-8-10
  12. Top Cat Breeds for 2004
  13. Preserving Quality and Genetic Diversity in a Dog Breed
  14. Kingston H M, "ABC of Clinical Genetics", Page 7, 3rd Edition (2002), BMJ Books, London, 0-7279-1627-0
  15. "The Habsburg Lip", Topics in the History of Genetics and Molecular Biology, Fall 2000
  16. Iceland's 'Kissing Cousins' Breed More Kids

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