Inbreeding is the sexual reproduction of offspring from the mating or breeding of individuals or organisms that are closely related genetically.  By analogy, the term is used in human reproduction, but more commonly refers to the genetic disorders and other consequences that may arise from incestuous sexual relationships and consanguinity.
Inbreeding results in homozygosity, which can increase the chances of offspring being affected by recessive or deleterious traits. This generally leads to a decreased biological fitness of a population (called inbreeding depression), which is its ability to survive and reproduce. An individual who inherits such deleterious traits is referred to as inbred. The avoidance of such deleterious recessive alleles caused by inbreeding, via inbreeding avoidance mechanisms, is the main selective reason for outcrossing. Crossbreeding between populations also often has positive effects on fitness-related traits.
Inbreeding is a technique used in selective breeding. In livestock breeding, breeders may use inbreeding when, for example, trying to establish a new and desirable trait in the stock, but will need to watch for undesirable characteristics in offspring, which can then be eliminated through further selective breeding or culling. Inbreeding is used to reveal deleterious recessive alleles, which can then be eliminated through assortative breeding or through culling. 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.
Offspring of biologically related persons are subject to the possible impact of inbreeding, such as congenital birth defects. The chances of such disorders is increased the closer the relationship of the biological parents. (See coefficient of inbreeding.) This is because such pairings increase the proportion of homozygous zygotes in the offspring, in particular deleterious recessive alleles, which produce such disorders. (See inbreeding depression.) Because most recessive alleles are rare in populations, it is unlikely that two unrelated marriage partners will both be carriers of the alleles. However, because close relatives share a large fraction of their alleles, the probability that any such deleterious allele is inherited from the common ancestor through both parents is increased dramatically. Contrary to common belief, inbreeding does not in itself alter allele frequencies, but rather increases the relative proportion of homozygotes to heterozygotes. However, because the increased proportion of deleterious homozygotes exposes the allele to natural selection, in the long run its frequency decreases more rapidly in inbred population. In the short term, incestuous reproduction is expected to produce increases in spontaneous abortions of zygotes, perinatal deaths, and postnatal offspring with birth defects. The advantages of inbreeding may be the result of a tendency to preserve the structures of alleles interacting at different loci that have been adapted together by a common selective history.
Malformations or harmful traits can stay within a population due to a high homozygosity rate and it will cause a population to become fixed for certain traits, like having too many bones in an area, like the vertebral column in wolves on Isle Royale or having cranial abnormalities in Northern elephant seals, where their cranial bone length in the lower mandibular tooth row has changed. Having a high homozygosity rate is bad for a population because it will unmask recessive deleterious alleles generated by mutations, reduce heterozygote advantage, and it is detrimental to the survival of small, endangered animal populations. When there are deleterious recessive alleles in a population it can cause inbreeding depression. The authors think that it is possible that the severity of inbreeding depression can be diminished if natural selection can purge such alleles from populations during inbreeding. If inbreeding depression can be diminished by natural selection than some traits, harmful or not, can be reduced and change the future outlook on a small, endangered populations.
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).
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 inbreeding depression becoming less severe. This is associated with the unmasking and eliminating of severely deleterious recessive alleles. It is not likely, though, that eliminating can be so complete that inbreeding depression is only a temporary phenomenon. 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 genomes accumulate deleterious mutations that are irreversible.
Autosomal recessive disorders occur in individuals who have two copies of the gene for a particular recessive genetic mutation. Except in certain rare circumstances, such as new mutations or uniparental disomy, both parents of an individual with such a disorder will be carriers of the gene. These carriers do not display any signs of the mutation and may be unaware that they carry the mutated gene. Since relatives share a higher proportion of their genes than do unrelated people, it is more likely that related parents will both be carriers of the same recessive gene, and therefore their children are at a higher risk of a genetic disorder. The extent to which the risk increases depends on the degree of genetic relationship between the parents: The risk is greater when the parents are close relatives and lower for relationships between more distant relatives, such as second cousins, though still greater than for the general population. A study has provided the evidence for inbreeding depression on cognitive abilities among children, with high frequency of mental retardation among offspring in proportion to their increasing inbreeding coefficients.
Children of parent-child or sibling-sibling unions are at increased risk compared to cousin-cousin unions.
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
- Depression on growth rate (height, weight and body mass index)
- 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 with 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 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.
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.
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 sexes, 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.
Although there are several examples of inbred populations of wild animals, the negative consequences of this inbreeding are poorly documented.
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 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 by 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 trophic 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 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.
In Central California, Sea Otters were thought to have been driven to extinction due to over hunting, until a colony of about 30 breeding pairs was discovered in the Big Sur region in the 1930s. Since then, the population has grown and spread along the central Californian coast to around 2,000 individuals, a level that has remained stable for over a decade. Population growth is limited by the fact that all Californian Sea Otters are descended from the isolated colony, resulting in inbreeding.
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.
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.
Measures of inbreeding
A measure of inbreeding of an individual A is the probability F(A) that both alleles in one locus are derived from the same gene in an ancestor. Two alleles derived from the same gene in an ancestor are said to be identical by descent. This probability F(A) is called the "coefficient of inbreeding".
Another useful measure that describes the extent to which two individuals are relatives (say individuals A and B) is their coancestry coefficient f(A,B), which gives the probability that, taking one random allele from A and another random allele from B, both are identical by descent. This is also denoted kinship coefficient between A and B.
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.
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 or are limited to neutral alleles.
Typical coancestries between relatives are as follows:
- Father/daughter, mother/son or 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)
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, through homozygosity, exposes recessive alleles.
Breeders must cull unfit breeding suppressed individuals 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 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. This method can be used to increase a particular animal's contribution to the population. 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 genepool that may include an increased prevalence of genetic disorders and inbreeding depression.
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. 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.
Systematic inbreeding and maintenance of inbred strains of laboratory mice and rats is of great importance for biomedical research. The inbreeding guarantees a consistent and uniform animal model for experimental purposes and enables genetic studies in congenic and knock-out animals. 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.
Effects of Inbreeding in Humans
Consanguineous relations have been tabooed for centuries and for good reason. The partners in these relationships that inbreed, simultaneously jeopardize their offspring's fitness and bring upon them great evolutionary risk. Inbreeding opens the door for prevalence of deleterious recessive alleles by increasing their chance for homozygosity and thereby, impeding the fitness of the offspring. With continuous inbreeding, the loss of genetic variation magnifies, and in turn, enables the expression of such recessive alleles following boundless negative implications. When determining the degree of inbreeding in an individual, scientists use a term called the inbreeding coefficient which estimates the percent of homozygous alleles in the overall genome. The more biologically related the inbreeders are, the greater the inbreeding coefficient (See Coefficient of Inbreeding), being that 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. 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.
Inbreeding is especially dangerous in small populations where the genetic variation is already limited. By inbreeding, individuals are further limiting the variation of alleles and ensure a higher percentage of homozygosity of their offspring's genome. Thus, the likelihood of deleterious recessive alleles to pair is significantly higher in an small inbreeding population than in a larger inbreeding population. By consistently increasing the homozygosity of alleles, the population is, in turn, allowing these harmful alleles to dominate, leading to expression of genetic disorders. Due to the inflicted individuals decreased fitness and reproductive success, the recessive alleles will eventually become culled by natural selection. Thus in small inbreeding populations, deleterious recessive alleles are more likely to be prevalent short term, but will decrease more swiftly long term after consistent expression followed by natural selection.
The fitness consequences of consanguineous mating have been studied since their scientific recognition by Charles Darwin in 1839. Some of the most harmful effects known from such breeding include its effects on the mortality rate, as well as the general health of the offspring. Within the past several decades, there have been many studies to support such debilitating effects on the human organism. Specifically, inbreeding has been found to decrease fertility as a direct result of increasing homozygosity of deleterious recessive alleles. Fetuses produced by inbreeding also face a greater risk of spontaneous abortions due to inherent complications in development. 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. Additionally, consanguineous parents possess a high risk of premature birth and producing underweight and undersized infants. Viable inbred offspring are also likely to be inflicted with physical deformities and genetically inherited diseases. Studies have confirmed an increase in several genetic disorders due to inbreeding such as blindness, hearing loss, neonatal diabetes, limb malformations, Schizophrenia and several others. 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. A study in 2009 revealed that inbred individuals have a greater susceptibility to TB and hepatitis. Being that resistance to these diseases is dependent on one's immune system, it is thereby suggested that consanguinity increases individual's susceptibility to infectious diseases. While inbreeding may seem to mainly implicate physical disparities, it also strongly affects cognitive ability. A recent behavioral study discovered that parental consanguinity followed by inbreeding is significantly correlated with the depression in intelligence among children. Furthermore, such offspring are known to have a lower IQ and higher risk for mental retardation.
Prevalence of Inbreeding
The general negative outlook and eschewal of inbreeding that is prevalent in the western world today holds roots from over 1500 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. This continuity 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. Of the practicing regions, Middle Eastern and northern Africa territories show the greatest frequencies of consanguinity. The link between the high frequency and the region is primarily due to the dominance of Islamic populations, who have historically engaged in familyline relations.
- Qatar- Historically, populations of Qatar have engaged in consanguineous relationships of all kinds, leading to high risk of inheriting genetic diseases. In 2014, a published study discovered that 5.2% of the Qatari population suffers from hereditary hearing loss. Reports illustrate that of the affected individuals, the majority were descendants of a consanguineous relationship. Even more so, molecular testing confirmed a high level of homogeneity and inbreeding coefficient among patients suffering from hereditary hearing loss.
- Iran- A recent study done on the prevalence of consanguinity in Iran determined that about 37% of the marriages were consanguineous with the majority being among rural couples, women marrying at a younger age, older marriage cohorts and women with lower levels of education. Thus, these results show a significant correlation between prevalence of inbreeding and low levels of education/limited access to education. However, since the nation is progressing towards prompt urbanization and expanding educational and employment opportunities, the frequency of inbreeding should be on a decline for future generations. Thereby, decreasing the incidence of recessive genetic disorders and promoting genetic diversity.
- Afghanistan & Lebanon- Populations within Afghanistan and Lebanon have also showed elevated levels of consanguinity. Studies determined that in Afghanistan, inbreeding was more heavily practiced in the more rural, western and southern regions. In Lebanon, a study focused on the dominating Shi'a populations and ascertained that about 28% of the marriages were consanguineous.
- Turkey- In 2009, a study of 200 full term infants and their families in Turkey established that the infants of consanguineous marriages weighed less and were of smaller size than the non-inbred infants. Moreover, of the 200 families studied, those that were consanguineous were found to be far less educated. Thus, parental education plays a role in the frequency of inbreeding. This would explain why western, generally more educated countries, inbreed far less than in the developing world.
- Israel- The Bedouin population in Israel has commonly practiced consanguinity throughout their existence, with studies illustrating its prevalence in modern day still. In recent years, of the Bedouin women studied, 45% of the women have been found to be in a consanguineous relationship of sort, with first cousin marriages the most common. The level of education and age of marriage were found to be significantly correlated with consanguinity, with lower levels of education and younger age of marriage influencing higher bloodline marriage rates. Evidently, efforts should be made to better educate the population on the repercussions of inbreeding in hopes to decrease their risk of congenital malformations and genetic diseases. While rates of inbreeding may still be high, those of Israeli Arabs have been decreasing. A 2010 study analyzed the rate of consanguineous marriages among Israeli Arabs from 2000-2009 and ascertained a steady decline from roughly 36% to 24%. Similar to the Bedouin population, most of the women who engaged in consanguinity married at a younger age, thus premarital counseling is highly advised to prevent such statistics.
Aside from the Middle East, north African territories such as Tunisia and Egypt experience similar consanguinity frequencies and correlations.
- Tunisia- Specifically, studies in 2006 found that among a Tunisian population examined, about 20% of the individuals were inbred. There was also a consistent correlation of increased frequency of consanguineous relations with a lower education level and occupation status.
- Egypt- In Egypt, researchers have observed high levels of inbreeding among populations, with consanguinity frequencies as high as 38% in rural regions. Moreover, a study done in 2010 concluded a link between bipolar I disorder and Schizophrenia, two genetic disorders prevalent in Egypt, and consanguinity.
Among these populations with high levels of inbreeding, researchers have found several disorders prevalent among inbred offspring. Specifically, in Lebanon, Saudi Arabia, Egypt, and Arabs in Israel, it has been discovered that offspring of consanguineous relationships have an increased risk of congenital malformations, congenital heart defects, congenital hydrocephalus and neural tube defects. Furthermore, among inbred children in Palestine and Lebanon, there is a positive association between consanguinity and reported cleft lip/palate cases. For further information on inbreeding effects, explore Tadmouri's review, "Consanguinity and reproductive health among Arabs," Bittle's article, "Consanguinity, human evolution, and complex diseases," and Hamamy's review, "Consanguineous marriages."
Possible increase of fertility
A study in Iceland by the deCODE genetics company, published by the journal Science, found that third cousins produced more children and grandchildren than more distant marriages, suggesting that "in spite of the fact that bringing together two alleles of a recessive trait may be bad, there may be some biological wisdom in the union of relatively closely related people". For hundreds of years, inbreeding was historically unavoidable in Iceland due to its then tiny and isolated population.
Royalty and nobility
Inter-nobility marriage was used as a method of forming 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, not of a contract between individuals as it is seen today.
Royal intermarriage was often practiced among European royal families, usually for interests of state. Over time, due to the relatively limited number of potential consorts, the gene pool of many ruling families grew progressively smaller, until all European royalty was related. This also resulted in many being descended from a certain person through many lines of descent, such as the numerous European royalty and nobility descended from the British Queen Victoria or King Christian IX of Denmark. The House of Habsburg was infamous for its inbreeding, with the Habsburg lip cited as an ill-effect, although no genetic evidence has proved the allegation. 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. Examples of incestuous marriages and the impact of inbreeding on royal families include:
- In ancient Egypt, royal women were believed to carry the bloodlines and so it was advantageous for a pharaoh to marry his sister or half-sister; 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 (also called Cleopatra VI) and Ptolemy XIII, who married and became co-rulers of ancient Egypt following their father's death, are the most widely known example.
- Among European monarchies, Jean V of Armagnac formed a rare brother-sister relationship. Also, other royal houses, such as the Wittelsbachs, had marriages among aunts, uncles, nieces and nephews.
- One of the most famous examples of a genetic trait aggravated by royal family intermarriage was the House of Habsburg, which inmarried particularly often and is known for the mandibular prognathism of the Habsburger (Unter) Lippe (otherwise known as the 'Habsburg jaw', 'Habsburg lip' or 'Austrian lip'"). This was 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.
- 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 led to the extinction of the dynasty.
- Francis II from the house of Habsburg-Lorraine married his double cousin Maria Theresa of Naples and Sicily, and several of their children may have had genetic health problems. Their daughter Marie Anne is said to have suffered from a hideous facial deformity and also to have been mentally deficient. Their son Ferdinand, who became an emperor, was also mentally deficient and suffered from Hydrocephalus (meaning water head), which resulted in an enlarged head. He also had several seizures daily. Of course he was never capable of leading the empire and relied on others and abdicated during the difficulties of the Revolutions of 1848. When informed of the revolution, he supposedly asked "But are they allowed to do that?" (Viennese German: Ja, dürfen's denn des?) Five of Francis II's children died in infancy or early childhood.
- Another famous genetic disease that circulated among European royalty was hemophilia. This spread to the royal families of Russia and Spain, and was a factor in the overthrow of both. Because the progenitor, Queen Victoria, was in a first cousin marriage, it is often believed that the cause was consanguinity (inbreeding). However, this disease (in males) is generally not aggravated by cousin marriages, although rare cases of hemophilia in girls can occur from the union of hemophiliac men and their cousins.
- 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 Chakri Dynasty of Thailand has included marriages between cousins as well as closer relatives. The current king, Bhumibol Adulyadej, is a first-cousin once removed of his wife, Sirikit, the two being respectively a grandson and a great-granddaughter of Chulalongkorn. The parents of the king's father, Mahidol Adulyadej, were half-siblings, both being children of Mongkut by different mothers.
- The Bone rank system in the Silla dynasty of Korea was structured so that the royal blood is passed down through the generations only if both parents were royalty. This 'sacred bone' caste system was so strictly enforced that it led to the crowning of Queen Seondeok, the first ruling queen in Korean history. This was only made possible because, by the time of Queen Seondoek's reign, all male 'sacred bones' have died off.
- In the British royal family, marriages to other royals or the nobility and marriages between close relatives, even to first-cousins, was the norm until the generation of the children of the present monarch, Elizabeth II. Elizabeth and her husband Prince Philip, Duke of Edinburgh are third cousins as a result of both being directly descended from Queen Victoria; as well as second cousins once removed as a result of being directly descended from Christian IX of Denmark. Queen Victoria herself had married her first cousin, Prince Albert of Saxe-Coburg and Gotha. Their nine children were married into royal and noble families across the continent, tying them together and earning her the nickname "the grandmother of Europe".
Today, royal intermarriage within European royal families has declined compared to past practice along with the power and prevalence of noble families and their importance in international affairs.
- Genetic diversity
- Coefficient of relationship
- Cousin marriage
- Evolution of sexual reproduction
- Genetic purging
- Genetic sexual attraction
- Heterozygote advantage
- Identical ancestors point
- Inbreeding depression
- Insular dwarfism
- Intellectual inbreeding
- List of coupled cousins
- Outbreeding depression
- Prohibited degree of kinship
- Selective breeding
- Self-incompatibility in plants (how some plants avoid inbreeding)
- Inbreeding at Encyclopædia Britannica
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