Inbreeding is the production of

offspring In biology, offspring are the young creation of living organisms, produced either by a single organism or, in the case of sexual reproduction, two organisms. Collective offspring may be known as a brood or progeny in a more general way. This ca ...

from the

mating In biology, mating is the pairing of either opposite- sex or hermaphroditic organisms for the purposes of sexual reproduction. ''Fertilization'' is the fusion of two gametes. ''Copulation'' is the union of the sex organs of two sexually reprod ...

or breeding of individuals or

organism In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells ( cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and fu ...

s that are closely related genetically. By analogy, the term is used in

human reproduction Human reproduction is sexual reproduction that results in human fertilization to produce a human offspring. It typically involves sexual intercourse between a sexually mature human male and female. During sexual intercourse, the interaction ...

, but more commonly refers to the genetic disorders and other consequences that may arise from expression of

deleterious {{Short pages monitor *

Banded mongoose The banded mongoose (''Mungos mungo'') is a mongoose species native from the Sahel to Southern Africa. It lives in savannas, open forests and grasslands and feeds primarily on beetles and millipedes. Mongooses use various types of dens for shelt ...

females regularly mate with their fathers and brothers. *

Bed bug Bed bugs are insects from the genus ''Cimex'' that feed on blood, usually at night. Their bites can result in a number of health impacts including skin rashes, psychological effects, and allergic symptoms. Bed bug bites may lead to skin changes ...

North Carolina State University North Carolina State University (NC State) is a public land-grant research university in Raleigh, North Carolina. Founded in 1887 and part of the University of North Carolina system, it is the largest university in the Carolinas. The univers ...

found that bedbugs, in contrast to most other insects, tolerate incest and are able to genetically withstand the effects of inbreeding quite well. *

Common fruit fly ''Drosophila melanogaster'' is a species of fly (the taxonomic order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly" or "pomace fly". Starting with ...

females prefer to mate with their own brothers over unrelated males. *

Cottony cushion scale ''Icerya purchasi'' (common name: cottony cushion scale) is a scale insect that feeds on more than 80 families of woody plants, most notably on ''Citrus'' and ''Pittosporum''. Originally described in 1878 from specimens collected in New Zealand a ...

s: '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.' * '' Adactylidium'': The single male offspring mite mates with all the daughters when they are still in the mother. The females, now impregnated, cut holes in their mother's body so that they can emerge to find new thrips eggs. The male emerges as well, but does not look for food or new mates, and dies after a few hours. The females die at the age of 4 days, when their own offspring eat them alive from the inside. White tiger-Gunma Safari Park

Domestic animals

Shaded silver Persian Cat Missionhill Cosmic Rainstorm

Intense inbreeding - Continuous sire to daughter mating

Breeding in domestic animals is primarily assortative breeding (see

selective breeding Selective breeding (also called artificial selection) is the process by which humans use animal breeding and plant breeding to selectively develop particular phenotypic traits (characteristics) by choosing which typically animal or plant ...

). 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. Breeders must avoid breeding from individuals that demonstrate either homozygosity or heterozygosity for disease causing alleles. The goal of preventing the transfer of deleterious alleles may be achieved by reproductive isolation,

sterilization Sterilization may refer to: * Sterilization (microbiology), killing or inactivation of micro-organisms * Soil steam sterilization, a farming technique that sterilizes soil with steam in open fields or greenhouses * Sterilization (medicine) rende ...

, or, in the extreme case,

culling In biology, culling is the process of segregating organisms from a group according to desired or undesired characteristics. In animal breeding, it is the process of removing or segregating animals from a breeding stock based on a specific tr ...

. 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. 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.

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. 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 gene pool that may include an increased prevalence of genetic disorders and inbreeding depression.

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 population genetics, the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population. It was first fully outlined by Ernst Mayr in 1942, us ...

. 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.

Laboratory animals

Systematic inbreeding and maintenance of inbred strains of

laboratory mice The laboratory mouse or lab mouse is a small mammal of the order Rodentia which is bred and used for scientific research or feeders for certain pets. Laboratory mice are usually of the species ''Mus musculus''. They are the most commonly used ...

and rats is of great importance for biomedical research. The inbreeding guarantees a consistent and uniform

animal model An animal model (short for animal disease model) is a living, non-human, often genetic-engineered animal used during the research and investigation of human disease, for the purpose of better understanding the disease process without the risk of ha ...

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. 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

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. With continuous inbreeding, genetic variation is lost and homozygosity is increased, enabling the expression of recessive deleterious alleles in homozygotes. The

coefficient of inbreeding The coefficient of inbreeding of an individual is the probability that two alleles at any locus in an individual are identical by descent from the common ancestor(s) of the two parents. The coefficient of inbreeding is: The probability ...

, or the degree of inbreeding in an individual, is an estimate of the percent of homozygous alleles in the overall genome. 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. 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. 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. Inbreeding is especially problematic in small populations where the genetic variation is already limited. By inbreeding, individuals are further decreasing genetic variation by increasing homozygosity in the genomes of their offspring. Thus, the likelihood of deleterious recessive alleles to pair is significantly higher in a small inbreeding population than in a larger inbreeding population. The fitness consequences of consanguineous mating have been studied since their scientific recognition by

Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended ...

in 1839. 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. Since the 1960s, 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,

disorders of sex development Disorders of sex development (DSDs), also known as differences in sex development, diverse sex development and variations in sex characteristics (VSC), are congenital conditions affecting the reproductive system, in which development of chromoso ...

, schizophrenia and several others. Moreover, there is an increased risk for congenital heart disease depending on the inbreeding coefficient (See

) of the offspring, with significant risk accompanied by an F =.125 or higher.

Prevalence

The general negative outlook and eschewal of inbreeding that is prevalent in the

Western world The Western world, also known as the West, primarily refers to the various nations and states in the regions of Europe, North America, and Oceania.

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. In the Western world some

Anabaptist Anabaptism (from Neo-Latin , from the Greek : 're-' and 'baptism', german: Täufer, earlier also )Since the middle of the 20th century, the German-speaking world no longer uses the term (translation: "Re-baptizers"), considering it biased. ...

groups are highly inbred because they originate from small founder populations and until today marriage outside the groups is not allowed for members. Especially the Reidenbach Old Order Mennonites and the

Hutterites Hutterites (german: link=no, Hutterer), also called Hutterian Brethren (German: ), are a communal ethnoreligious branch of Anabaptists, who, like the Amish and Mennonites, trace their roots to the Radical Reformation of the early 16th cent ...

stem from very small founder populations. The same is true for some

Hasidic Hasidism, sometimes spelled Chassidism, and also known as Hasidic Judaism ( Ashkenazi Hebrew: חסידות ''Ḥăsīdus'', ; originally, "piety"), is a Jewish religious group that arose as a spiritual revival movement in the territory of conte ...

and Haredi Jewish groups. Of the practicing regions, Middle Eastern and northern Africa territories show the greatest frequencies of consanguinity. Among these populations with high levels of inbreeding, researchers have found several disorders prevalent among inbred offspring. In

Lebanon Lebanon ( , ar, لُبْنَان, translit=lubnān, ), officially the Republic of Lebanon () or the Lebanese Republic, is a country in Western Asia. It is located between Syria to the north and east and Israel to the south, while Cyprus lie ...

Saudi Arabia Saudi Arabia, officially the Kingdom of Saudi Arabia (KSA), is a country in Western Asia. It covers the bulk of the Arabian Peninsula, and has a land area of about , making it the fifth-largest country in Asia, the second-largest in the Ara ...

Egypt Egypt ( ar, مصر , ), officially the Arab Republic of Egypt, is a List of transcontinental countries, transcontinental country spanning the North Africa, northeast corner of Africa and Western Asia, southwest corner of Asia via a land bridg ...

, and in

Israel Israel (; he, יִשְׂרָאֵל, ; ar, إِسْرَائِيل, ), officially the State of Israel ( he, מְדִינַת יִשְׂרָאֵל, label=none, translit=Medīnat Yīsrāʾēl; ), is a country in Western Asia. It is situated ...

, the offspring of consanguineous relationships have an increased risk of congenital malformations, congenital heart defects, congenital

hydrocephalus Hydrocephalus is a condition in which an accumulation of cerebrospinal fluid (CSF) occurs within the brain. This typically causes increased pressure inside the skull. Older people may have headaches, double vision, poor balance, urinary i ...

and

neural tube defect Neural tube defects (NTDs) are a group of birth defects in which an opening in the spine or cranium remains from early in human development. In the third week of pregnancy called gastrulation, specialized cells on the dorsal side of the embryo b ...

s. Furthermore, among inbred children in Palestine and Lebanon, there is a positive association between consanguinity and reported

cleft lip/palate A cleft lip contains an opening in the upper lip that may extend into the nose. The opening may be on one side, both sides, or in the middle. A cleft palate occurs when the palate (the roof of the mouth) contains an opening into the nose. The te ...

cases. Historically, populations of

Qatar Qatar (, ; ar, قطر, Qaṭar ; local vernacular pronunciation: ), officially the State of Qatar,) is a country in Western Asia. It occupies the Qatar Peninsula on the northeastern coast of the Arabian Peninsula in the Middle East; it sh ...

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.

Royalty and nobility

Inter-nobility

marriage Marriage, also called matrimony or wedlock, is a culturally and often legally recognized union between people called spouses. It establishes rights and obligations between them, as well as between them and their children, and between ...

was used as a method of forming

political alliance A political group is a group consisting of political parties or legislators of aligned ideologies. A technical group is similar to a political group, but with members of differing ideologies. International terms Equivalent terms are used differ ...

s among elites. These ties were often sealed only upon the birth of progeny within the

arranged marriage Arranged marriage is a type of marital union where the bride and groom are primarily selected by individuals other than the couple themselves, particularly by family members such as the parents. In some cultures a professional matchmaker may be us ...

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

Royal intermarriage Royal intermarriage is the practice of members of ruling dynasties marrying into other reigning families. It was more commonly done in the past as part of strategic diplomacy for national interest. Although sometimes enforced by legal requiremen ...

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 The gene pool is the set of all genes, or genetic information, in any population, usually of a particular species. Description A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can surv ...

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 Victoria (Alexandrina Victoria; 24 May 1819 – 22 January 1901) was Queen of the United Kingdom of Great Britain and Ireland from 20 June 1837 until her death in 1901. Her reign of 63 years and 216 days was longer than that of any previ ...

or King Christian IX of Denmark. The House of

Habsburg The House of Habsburg (), alternatively spelled Hapsburg in Englishgerman: Haus Habsburg, ; es, Casa de Habsburgo; hu, Habsburg család, it, Casa di Asburgo, nl, Huis van Habsburg, pl, dom Habsburgów, pt, Casa de Habsburgo, la, Domus Hab ...

was known for its intermarriages; the Habsburg lip often cited as an ill-effect. The closely related houses of Habsburg, Bourbon, Braganza and

Wittelsbach The House of Wittelsbach () is a German dynasty, with branches that have ruled over territories including Bavaria, the Palatinate, Holland and Zeeland, Sweden (with Finland), Denmark, Norway, Hungary (with Romania), Bohemia, the Electorate ...

also frequently engaged in first-cousin unions as well as the occasional double-cousin and uncle–niece marriages. In ancient Egypt, royal women were believed to carry the bloodlines and so it was advantageous for a

pharaoh Pharaoh (, ; Egyptian: '' pr ꜥꜣ''; cop, , Pǝrro; Biblical Hebrew: ''Parʿō'') is the vernacular term often used by modern authors for the kings of ancient Egypt who ruled as monarchs from the First Dynasty (c. 3150 BC) until th ...

to marry his sister or half-sister; in such cases a special combination between

endogamy Endogamy is the practice of marrying within a specific social group, religious denomination, caste, or ethnic group, rejecting those from others as unsuitable for marriage or other close personal relationships. Endogamy is common in many cultu ...

and

polygamy Crimes Polygamy (from Late Greek (') "state of marriage to many spouses") is the practice of marrying multiple spouses. When a man is married to more than one wife at the same time, sociologists call this polygyny. When a woman is marr ...

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 The Ptolemaic dynasty (; grc, Πτολεμαῖοι, ''Ptolemaioi''), sometimes referred to as the Lagid dynasty (Λαγίδαι, ''Lagidae;'' after Ptolemy I's father, Lagus), was a Macedonian Greek royal dynasty which ruled the Ptolemaic ...

uninterruptedly from

Ptolemy IV egy, Iwaennetjerwymenkhwy Setepptah Userkare Sekhemankhamun Clayton (2006) p. 208. , predecessor = Ptolemy III , successor = Ptolemy V , horus = ''ḥnw-ḳni sḫꜤi.n-sw-it.f'Khunuqeni sekhaensuitef'' The strong youth whose f ...

(

Ptolemy II ; egy, Userkanaenre Meryamun Clayton (2006) p. 208 , predecessor = Ptolemy I , successor = Ptolemy III , horus = ''ḥwnw-ḳni'Khunuqeni''The brave youth , nebty = ''wr-pḥtj'Urpekhti''Great of strength , gold ...

married his sister but had no issue) were married to their brothers and sisters, so as to keep the Ptolemaic blood "pure" and to strengthen the line of succession. King

Tutankhamun Tutankhamun (, egy, twt-ꜥnḫ-jmn), Egyptological pronunciation Tutankhamen () (), sometimes referred to as King Tut, was an Egyptian pharaoh who was the last of his royal family to rule during the end of the Eighteenth Dynasty (ruled ...

's mother is reported to have been the half-sister to his father,

Cleopatra VII Cleopatra VII Philopator ( grc-gre, Κλεοπάτρα Φιλοπάτωρ}, "Cleopatra the father-beloved"; 69 BC10 August 30 BC) was Queen of the Ptolemaic Kingdom of Egypt from 51 to 30 BC, and its last active ruler.She was also a ...

(also called Cleopatra VI) and

Ptolemy XIII Ptolemy XIII Theos Philopator ( grc-gre, Πτολεμαῖος Θεός Φιλοπάτωρ, ''Ptolemaĩos''; c. 62 BC – 13 January 47 BC) was Pharaoh of Egypt from 51 to 47 BC, and one of the last members of the Ptolemaic dynasty (305–30 BC) ...

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

References

External links

Dale Vogt, Helen A. Swartz and John Massey, 1993. ''Inbreeding: Its Meaning, Uses and Effects on Farm Animals''. University of Missouri, Extension.

Consanguineous marriages with global map
* {{Incest Population genetics Breeding Incest Kinship and descent