Inbreeding is the production of

offspring In biology, offspring are the young creation of living organisms, produced either by sexual reproduction, sexual or asexual reproduction. Collective offspring may be known as a brood or progeny. This can refer to a set of simultaneous offspring ...

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 repr ...

or breeding of individuals or

organism An organism is any life, living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have be ...

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

human reproduction Human sexual reproduction, to produce offspring, begins with fertilization. Successful reproduction typically involves sexual intercourse between a healthy, sexually mature and fertile male and female. During sexual intercourse, sperm cells are ...

, 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 parasitic insects from the genus ''Cimex'', which are micropredators that haematophagy, feed on blood, usually at night. Their bites can result in a number of health impacts, including skin rashes, psychological effects, and aller ...

North Carolina State University North Carolina State University (NC State, North Carolina State, NC State University, or NCSU) is a public university, public Land-grant university, land-grant research university in Raleigh, North Carolina, United States. Founded in 1887 and p ...

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

Adactylidium ''Adactylidium'' is a genus of mites known for its unusual life cycle. An impregnated female mite feeds upon a single egg of a thrips Thrips (Order (biology) , order Thysanoptera) are minute (mostly long or less), slender insects with fr ...

'': 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. 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 m ...

). 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, or, in the extreme case,

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

. 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.

As a result of long-term cooperation between

USDA The United States Department of Agriculture (USDA) is an United States federal executive departments, executive department of the Federal government of the United States, United States federal government that aims to meet the needs of commerc ...

and dairy farmers which led to a revolution in dairy cattle productivity, the United States has since 1992 been the world’s largest supplier of dairy bull semen. However, US genomic technology has resulted in the US dairy cattle population becoming "the most inbred it’s ever been" and the rate of increase in US national milk yield has tapered off. Efforts are now being made to identify desirable genes in cattle breeds not yet optimized by US dairy breeders in order to apply

hybrid vigor Heterosis, hybrid vigor, or outbreeding enhancement is the improved or increased function of any biological quality in a Hybrid (biology), hybrid offspring. An offspring is heterotic if its trait (biology), traits are enhanced as a result of m ...

to the US dairy cattle population and thus propel US dairy technology to even higher levels of productivity. The BBC produced two documentaries on dog inbreeding titled ''

Pedigree Dogs Exposed ''Pedigree Dogs Exposed'' is a BBC One investigative documentary, produced by Jemima Harrison, which looks into health and welfare issues facing pedigree dogs in the United Kingdom. It was originally broadcast on 19 August 2008. The Kennel Club ...

'' 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 animal sources for these mice are usually of the species ''Mus musculus''. They a ...

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. Depending on the rate of inbreeding, natural selection may still be able to eliminate deleterious alleles. 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. 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 Natural history#Before 1900, naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all speci ...

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, variations in sex characteristics (VSC), sexual anomalies, or sexual abnormalities, are congenital conditions affecting the reproductive system, in which developme ...

, 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.

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 various nations and state (polity), states in Western Europe, Northern America, and Australasia; with some debate as to whether those in Eastern Europe and Latin America also const ...

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 language, Greek : 're-' and 'baptism'; , 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 that have bred as a closed population. Of the practicing regions, Middle Eastern and northern African nations 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, officially the Republic of Lebanon, is a country in the Levant region of West Asia. Situated at the crossroads of the Mediterranean Basin and the Arabian Peninsula, it is bordered by Syria to the north and east, Israel to the south ...

Saudi Arabia Saudi Arabia, officially the Kingdom of Saudi Arabia (KSA), is a country in West Asia. Located in the centre of the Middle East, it covers the bulk of the Arabian Peninsula and has a land area of about , making it the List of Asian countries ...

Egypt Egypt ( , ), officially the Arab Republic of Egypt, is a country spanning the Northeast Africa, northeast corner of Africa and Western Asia, southwest corner of Asia via the Sinai Peninsula. It is bordered by the Mediterranean Sea to northe ...

, and

Israel Israel, officially the State of Israel, is a country in West Asia. It Borders of Israel, shares borders with Lebanon to the north, Syria to the north-east, Jordan to the east, Egypt to the south-west, and the Mediterranean Sea to the west. Isr ...

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

hydrocephalus Hydrocephalus is a condition in which cerebrospinal fluid (CSF) builds up within the brain, which can cause pressure to increase in the skull. Symptoms may vary according to age. Headaches and double vision are common. Elderly adults with n ...

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 ...

s. Furthermore, among inbred children in

Palestine Palestine, officially the State of Palestine, is a country in West Asia. Recognized by International recognition of Palestine, 147 of the UN's 193 member states, it encompasses the Israeli-occupied West Bank, including East Jerusalem, and th ...

and Lebanon, there is a positive association between consanguinity and reported cleft lip/palate cases. Historically, populations of

Qatar Qatar, officially the State of Qatar, is a country in West Asia. It occupies the Geography of Qatar, Qatar Peninsula on the northeastern coast of the Arabian Peninsula in the Middle East; it shares Qatar–Saudi Arabia border, its sole land b ...

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. In 2017-2019, congenital anomalies due to inbreeding was the most common cause of death of babies belonging to the Pakistani and Bangladeshi ethnic groups in England and Wales.

Royalty and nobility

Inter-nobility

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

was used as a method of forming

political alliance A parliamentary group, parliamentary caucus or political group is a group consisting of members of different political parties or independent politicians with similar ideologies. Some parliamentary systems allow smaller political parties, who a ...

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

arranged marriage Arranged marriage is a type of Marriage, 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 matchmaki ...

. 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 requirem ...

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 survi ...

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 Death and state funeral of Queen Victoria, her death in January 1901. Her reign of 63 year ...

King Christian IX of Denmark Christian IX (8 April 181829 January 1906) was King of Denmark from 15 November 1863 until his death in 1906. From 1863 to 1864, he was concurrently List of dukes of Schleswig, Duke of Schleswig, List of dukes of Holstein, Holstein and Saxe-Laue ...

. The House of

Habsburg The House of Habsburg (; ), also known as the House of Austria, was one of the most powerful dynasties in the history of Europe and Western civilization. They were best known for their inbreeding and for ruling vast realms throughout Europe d ...

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 former Bavarian dynasty, with branches that have ruled over territories including the Electorate of Bavaria, the Electoral Palatinate, the Electorate of Cologne, County of Holland, Holland, County of Zeeland, ...

also frequently engaged in first-cousin unions as well as the occasional double-cousin and uncle–niece marriages. In

ancient Egypt Ancient Egypt () was a cradle of civilization concentrated along the lower reaches of the Nile River in Northeast Africa. It emerged from prehistoric Egypt around 3150BC (according to conventional Egyptian chronology), when Upper and Lower E ...

, royal women were believed to carry the bloodlines and so it was advantageous for a

pharaoh Pharaoh (, ; Egyptian language, Egyptian: ''wikt:pr ꜥꜣ, pr ꜥꜣ''; Meroitic language, Meroitic: 𐦲𐦤𐦧, ; Biblical Hebrew: ''Parʿō'') was the title of the monarch of ancient Egypt from the First Dynasty of Egypt, First Dynasty ( ...

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

endogamy Endogamy is the cultural practice of marrying within a specific social group, religious denomination, caste, or ethnic group, rejecting any from outside of the group or belief structure as unsuitable for marriage or other close personal relatio ...

and

polygamy Polygamy (from Late Greek , "state of marriage to many spouses") is the practice of marriage, marrying multiple spouses. When a man is married to more than one wife at the same time, it is called polygyny. When a woman is married to more tha ...

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 (; , ''Ptolemaioi''), also known as the Lagid dynasty (, ''Lagidai''; after Ptolemy I's father, Lagus), was a Macedonian Greek royal house which ruled the Ptolemaic Kingdom in Ancient Egypt during the Hellenistic period. ...

uninterruptedly from Ptolemy IV (

Ptolemy II Ptolemy II Philadelphus (, ''Ptolemaîos Philádelphos'', "Ptolemy, sibling-lover"; 309 – 28 January 246 BC) was the pharaoh of Ptolemaic Egypt from 284 to 246 BC. He was the son of Ptolemy I, the Macedonian Greek general of Alexander the G ...

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 or Tutankhamen, (; ), was an Egyptian pharaoh who ruled during the late Eighteenth Dynasty of Egypt, Eighteenth Dynasty of ancient Egypt. Born Tutankhaten, he instituted the restoration of the traditional polytheistic form of an ...

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

Cleopatra VII Cleopatra VII Thea Philopator (; The name Cleopatra is pronounced , or sometimes in both British and American English, see and respectively. Her name was pronounced in the Greek dialect of Egypt (see Koine Greek phonology). She was ...

(also called Cleopatra VI) and

Ptolemy XIII Ptolemy XIII Theos Philopator (, ''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). He was the son of Ptolemy XII and the brother of and co ...

, who married and became co-rulers of

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