Inbreeding is the production 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 expression 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, but also sometimes
leads to negative effects known as outbreeding depression.
Inbreeding is a technique used in selective breeding. For example, in
livestock breeding, breeders may use inbreeding when 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.
2 Genetic disorders
4.1 Wild animals
4.2 Semi-domestic animals
4.3 Domestic animals
4.4 Laboratory animals
5.3 Royalty and nobility
6 See also
8 External links
Offspring of biologically related persons are subject to the possible
effects of inbreeding, such as congenital birth defects. The chances
of such disorders are increased when the biological parents are more
closely related. This is because such pairings have a 25% probability
of producing homozygous zygotes, resulting in offspring with two
recessive alleles, which can produce disorders when these alleles are
deleterious. Because most recessive alleles are rare in
populations, it is unlikely that two unrelated marriage partners will
both be carriers of the same deleterious allele; 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. It
should also be noted that for each homozygous recessive individual
formed there is an equal chance of producing a homozygous dominant
individual — one completely devoid of the harmful allele. 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 populations. In the short term, incestuous reproduction is
expected to increase the number of 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 this will cause a population to become
fixed for certain traits, like having too many bones in an area, like
the vertebral column of wolves on Isle Royale or having cranial
abnormalities, such as in Northern elephant seals, where their cranial
bone length in the lower mandibular tooth row has changed. Having a
high homozygosity rate is problematic 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 deleterious recessive
alleles are unmasked due to the increased homozygosity generated by
inbreeding, this can cause inbreeding depression.
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 that inbreeding depression becomes less severe.
This is associated with the unmasking and elimination of severely
deleterious recessive alleles. However, inbreeding depression is not a
temporary phenomenon because this elimination of deleterious recessive
alleles will never be complete. Eliminating slightly deleterious
mutations through inbreeding under moderate selection is not as
effective. Fixation of alleles most likely occurs through Muller's
ratchet, when an asexual population's genome accumulates deleterious
mutations that are irreversible.
Despite all its disadvantages, inbreeding can also have a variety of
advantages, such as reducing the recombination load, and allowing
the expression of recessive advantageous phenotypes. It has been
proposed that under circumstances when the advantages of inbreeding
outweigh the disadvantages, preferential breeding within small groups
could be promoted, potentially leading to speciation.
Animation of uniparental isodisomy
Autosomal recessive disorders occur in individuals who have two copies
of an allele 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 allele, and therefore their children are at a
higher risk of inheriting an autosomal recessive 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
Children of parent-child or sibling-sibling unions are at an increased
risk compared to cousin-cousin unions.
Inbreeding may result in a
greater than expected phenotypic expression of deleterious recessive
alleles within a population. As a result, first-generation inbred
individuals are more likely to show physical and health
Reduced fertility both in litter size and sperm viability
Increased genetic disorders
Fluctuating facial asymmetry
Lower birth rate
Higher infant mortality and child mortality
Smaller adult size
Loss of immune system function
Increased cardiovascular risks
The isolation of a small population for a period of time can lead to
inbreeding within that population, resulting in increased genetic
relatedness between breeding individuals.
Inbreeding depression can
also occur in a large population if individuals tend to mate with
their relatives, instead of mating randomly.
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
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
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
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 was responsible for most of the individuals
within the population. Even so, the diversity within the lines allowed
great variation in the gene pool that may help to protect the South
American sea lion from extinction.
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 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
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.
A measure of inbreeding of an individual A is the probability F(A)
that both alleles in one locus are derived from the same allele in an
ancestor. These two identical alleles that are both derived from a
common ancestor are said to be 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 related (say individuals A and B) is their coancestry
coefficient f(A,B), which gives the probability that one randomly
selected allele from A and another randomly selected allele from B are
identical by descent. This is also denoted as the 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 +
Both the inbreeding and the coancestry coefficients can be defined for
specific individuals or as average population values. They can be
computed from genealogies or estimated from the population size and
its breeding properties, but all methods assume no selection and are
limited to neutral alleles.
There are several methods to compute this percentage. The two main
ways are the path method and the tabular method.[unreliable
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-grandmother/great-grandson → 6.25% (1⁄16)
Half-uncle/niece or half-aunt/nephew → 6.25% (1⁄16)
First cousins → 6.25% (1⁄16)
Few studies have found evidence of regular incest in mammals but
Banded mongooses are an exception.
Banded mongoose females regularly mate with their fathers and
North Carolina State University
North Carolina State University found that bedbugs in
contrary to most other insects tolerate incest and are able to
genetically withstand the effects of inbreeding quite well; this is an
important biological discovery.
Common fruit fly
Common fruit fly females prefer to mate with their own brothers over
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: 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
White tiger in Gunma Safari Park
Hereditary polycystic kidney disease is prevalent in the Persian cat
breed, affecting almost half the population in some countries.
An intensive form of inbreeding where an individual S is mated to his
daughter D1, granddaughter D2 and so on, in order to maximise the
percentage of S's genes in the offspring. 87.5% of D3's genes would
come from S, while D4 would receive 93.75% of their genes from S.
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
Homozygosity is the case where similar or identical alleles
combine to express a trait that is not otherwise expressed
Inbreeding exposes recessive alleles through
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 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
Pedigree Dogs Exposed - Three Years On that document
the negative health consequences of excessive inbreeding.
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.[citation
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
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.
See also: Incest,
Incest taboo, Pedigree collapse, and Cousin marriage
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Inbreeding increases the chances of the expression of deleterious
recessive alleles by increasing homozygosity and therefore has the
potential to decrease 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, a term used to describe
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.
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
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 includes its
effects on the mortality rate as well as on 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
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
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. Historically, populations of
Qatar have engaged
in consanguineous relationships of all kinds, leading to high risk of
inheriting genetic diseases. As of 2014, around 5% of the Qatari
population suffered from hereditary hearing loss; most were
descendants of a consanguineous relationship.
Royalty and nobility
Main article: Royal intermarriage
See also: List of coupled cousins
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 as 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 uninterruptedly from
Ptolemy IV (
Ptolemy II 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. 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.
In Europe, the House of
Habsburg famously possessed the mandibular
prognathism of the Habsburger (Unter) Lippe (otherwise known as the
Habsburg jaw', '
Habsburg lip' or 'Austrian lip'"). This was typical
Habsburg relatives over a period of six centuries. Due to
multiple consanguineous marriages within the family, the condition
progressed through the generations until the last Spanish Habsburg,
Charles II of Spain, could not properly chew his food. Charles II
had a large number of genetic, physical, intellectual, sexual, and
emotional problems. His inbreeding coefficient was 0.254, equivalent
to the offspring of brother-sister incest. His
impotence/infertility led to the extinction of the Spanish Habsburgs
upon his death, triggering the War of the Spanish Succession.
Coefficient of relationship
Evolution of sexual reproduction
Genetic sexual attraction
Identical ancestors point
List of coupled cousins
Pedigree Dogs Exposed
Pedigree Dogs Exposed From forced inbreeding, dogs that cannot run,
breathe or see freely.
Pedigree Dogs Exposed: Three Years On From forced inbreeding, dogs
that cannot run, breathe or see freely.
Prohibited degree of kinship
Self-incompatibility in plants (how some plants avoid inbreeding)
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the British Isles
the Near East
List of genetics research organizations
Child incestuous abuse
in the Middle East
Incest between twins
Parallel and cross cousins
Laws regarding incest
Article 809 of the Korean Civil Code
Cousin marriage law in the United States by state
Cousin marriage court cases in the United States
Muth v. Frank
Laws regarding incest in the United States
Prohibited degree of kinship
Incest in the Bible
Incest in folklore
Incest in popular culture
Incest in film and television
Incest in literature
Jewish views on incest
Coefficient of relationship
Genetic sexual attraction
Colt clan incest case
Moe incest case