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Inbreeding depression is the reduced
biological fitness Fitness (often denoted w or ω in population genetics models) is the quantitative representation of natural and sexual selection File:Sexual Selection with Peafowl.gif, 250px, Sexual selection is a form of natural selection where one sex pref ...
in a given
population Population typically refers the number of people in a single area whether it be a city or town, region, country, or the world. Governments typically quantify the size of the resident population within their jurisdiction by a process called a ...

population
as a result of
inbreeding Inbreeding is the production of offspring In biology, offspring are the young born of living organism, organisms, produced either by a single organism or, in the case of sexual reproduction, two organisms. Collective offspring may be known as ...
(the breeding of related individuals). Population biological fitness refers to an organism's ability to survive and perpetuate its genetic material. Inbreeding depression is often the result of a
population bottleneck A population bottleneck or genetic bottleneck is a sharp reduction in the size of a due to environmental events such as famines, earthquakes, floods, fires, , and s; or human activities such as specicide, widespread violence or intentional cullin ...

population bottleneck
. In general, the higher the
genetic variation thumb File:Genetic Variation and Inheritance.svg, Parents have similar gene coding in this specific situation where they reproduce and variation in the offspring is seen. Offspring containing the variation also reproduce and passes down traits t ...

genetic variation
or
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 surviv ...
within a breeding population, the less likely it is to suffer from inbreeding depression. Inbreeding depression seems to be present in most groups of organisms, but varies across mating systems.
Hermaphroditic In reproductive biology, a hermaphrodite () is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life ...
species often exhibit lower degrees of inbreeding depression than
outcrossing Out-crossing or out-breeding is the technique of crossing between different breeds. This is the practice of introducing unrelated genetic material into a breeding line. It increases genetic diversity, thus reducing the probability of an individua ...
species, as repeated generations of
selfing Selfing or self-fertilization is the union of male Male (♂) is the sex of an organism that produces the gamete known as sperm. A male gamete can fuse with a larger female gamete, or ovum, in the process of fertilization. A male cannot sexual re ...
is thought to
purge In history History (from Greek , ''historia'', meaning "inquiry; knowledge acquired by investigation") is the study of the past. Events occurring before the invention of writing systems are considered prehistory. "History" is an umbrella term ...
deleterious
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is one of two, or more, forms of a given gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance ...
s from populations. For example, the outcrossing nematode (roundworm) ''
Caenorhabditis remanei ''Caenorhabditis remanei'' is a species of nematode found in North America and Europe, and likely lives throughout the temperateness, temperate world. Several strains have been developed in the laboratory.C. elegans
C. elegans
'', which experiences
outbreeding depressionIn biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, De ...
.


Mechanisms

Inbreeding (i.e., breeding between closely related individuals) results in more
recessive In genetics Genetics is a branch of biology concerned with the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) Though heredity had been observed for millennia, Gregor Mendel, Moravia, Moravian scientist ...
traits manifesting themselves, as the
genome In the fields of molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, M ...

genome
s of pair-mates are more similar. Recessive traits can only occur in an offspring if present in both parents' genomes. The more genetically similar the parents are, the more often recessive traits appear in their offspring. Consequently, the more closely related the breeding pair is, the more
homozygous Zygosity (the noun, zygote, is from the Greek zygotos "yoked," from zygon "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an or ...
, deleterious
gene In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mecha ...

gene
s the offspring may have, resulting in very unfit individuals. For
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is one of two, or more, forms of a given gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance ...
s that confer an advantage in the
heterozygous Zygosity (the noun, zygote, is from the Greek zygotos "yoked," from zygon "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an or ...

heterozygous
and/or homozygous-dominant state, the fitness of the homozygous-recessive state may even be zero (meaning sterile or unviable offspring). An example of inbreeding depression is shown to the right. In this case, ''a'' is the recessive allele which has negative effects. In order for the ''a'' phenotype to become active, the gene must end up as homozygous ''aa'' because in the geneotype A''a'', the A takes dominance over the ''a'' and the ''a'' does not have any effect. Due to their reduced phenotypic expression and their consequent reduced selection, recessive genes are, more often than not, detrimental phenotypes by causing the organism to be less fit to its natural environment. Another mechanism responsible for inbreeding depression is the fitness advantage of heterozygosity, which is known as
overdominance Overdominance is a condition in genetics where the phenotype of the Zygosity, heterozygote lies outside the phenotypical range of both Zygosity, homozygous parents. Overdominance can also be described as heterozygote advantage, wherein heterozygous ...
. This can lead to reduced fitness of a population with many homozygous genotypes, even if they are not deleterious or recessive. Here, even the dominant alleles result in reduced fitness if present homozygously (see also
hybrid vigour
hybrid vigour
). Currently, it is not known which of the two mechanisms is more prevalent in nature. For practical applications, e.g. in
livestock Livestock are the domesticated Domestication is a sustained multi-generational relationship in which one group of organisms assumes a significant degree of influence over the reproduction and care of another group to secure a more predictabl ...
breeding, the former is thought to be more significant – it may yield completely unviable offspring (meaning outright failure of a
pedigree Pedigree may refer to: Breeding * Pedigree chart, a document to record ancestry, used by genealogists in study of human family lines, and in selective breeding of other animals ** Pedigree, a human genealogy (ancestry chart) ** Pedigree (animal) ...
), while the latter can only result in relatively reduced fitness.


Natural selection

Natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of peta ...
cannot effectively remove all deleterious recessive genes from a population for several reasons. First, deleterious genes arise constantly through mutation within a population. Second, in a population where inbreeding occurs frequently, most offspring will have some deleterious traits, so few will be more fit for survival than the others. Different deleterious traits are extremely unlikely to equally affect reproduction – an especially disadvantageous recessive trait expressed in a homozygous recessive individual is likely to eliminate itself, naturally limiting the expression of its
phenotype In genetics Genetics is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular inter ...

phenotype
. Third, recessive deleterious alleles will be "masked" by heterozygosity, and so in a dominant-recessive trait, heterozygotes will not be selected against. When recessive deleterious alleles occur in the heterozygous state, where their potentially deleterious expression is masked by the corresponding wild-type allele, this masking phenomenon is referred to as complementation (see
complementation (genetics) In genetics Genetics is a branch of biology concerned with the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) Though heredity had been observed for millennia, Gregor Mendel, Moravia, Moravian scientist ...
). In general, sexual reproduction in
eukaryotes Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synonym for "Outline ...
has two fundamental aspects:
genetic recombination Genetic recombination (also known as genetic reshuffling) is the exchange of genetic material between different organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, ph ...
during
meiosis Meiosis (; , because it is a reductional division) is a special type of of in organisms used to produce the , such as or . It involves two rounds of division that ultimately result in four cells with only one copy of each (). Additionall ...

meiosis
, and
outcrossing Out-crossing or out-breeding is the technique of crossing between different breeds. This is the practice of introducing unrelated genetic material into a breeding line. It increases genetic diversity, thus reducing the probability of an individua ...
. It has been proposed that these two aspects have two natural selective advantages respectively. A proposed adaptive advantage of meiosis is that it facilitates recombinational repair of DNA damages that are otherwise difficult to repair (see DNA repair as the adaptive advantage of meiosis). A proposed adaptive advantage of outcrossing is complementation, which is the masking of deleterious recessive allelesMichod, R.E. Eros and Evolution: A Natural Philosophy of Sex. (1996) Perseus Books (see
hybrid vigor or heterosis
hybrid vigor or heterosis
). The selective advantage of complementation may largely account for the general avoidance of inbreeding (see
kin recognitionKin recognition, also called kin detection, is an organism's ability to distinguish between close genetic kin and non-kin. In evolutionary biology Evolutionary biology is the subfield of biology that studies the evolution, evolutionary processes ...
).


Management

Introducing alleles from a different population can reverse inbreeding depression. Different populations of the same species have different deleterious traits, and therefore their cross breeding will not result in
homozygosity Zygosity (the noun, zygote A zygote (from Greek ζυγωτός ''zygōtos'' "joined" or "yoked", from ζυγοῦν ''zygoun'' "to join" or "to yoke") is a eukaryotic cell formed by a fertilization event between two gamete A gamete ( /ˈɡ ...
at most
loci Locus (plural loci) is Latin for "place". It may refer to: Entertainment * Locus (comics), a Marvel Comics mutant villainess, a member of the Mutant Liberation Front * Locus (magazine), ''Locus'' (magazine), science fiction and fantasy magazine ...
in the offspring. This is known as
outbreeding enhancement
outbreeding enhancement
, practiced by conservation managers and zoo captive breeders to prevent homozygosity. However, intermixing two different populations can give rise to unfit polygenic traits in
outbreeding depressionIn biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, De ...
(i.e. yielding offspring which lack the genetic
adaptation In , adaptation has three related meanings. Firstly, it is the dynamic evolutionary process that fits s to their environment, enhancing their . Secondly, it is a state reached by the population during that process. Thirdly, it is a or adapti ...

adaptation
s to specific environmental conditions). These, then, will have a lowered fitness than pure-bred individuals of a particular
subspecies In biological classification In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interacti ...
that has adapted to its local environment.


In humans

The biological effects of inbreeding depression in humans are largely obscured by socioeconomic and cultural influences on reproductive behavior. Studies in human populations have shown that age at
marriage Marriage, also called matrimony or wedlock is a culturally and often legally recognized union between people called spouse A religious marriage. A spouse is a significant other Significant other (SO) is colloquially used as a term ...

marriage
, duration of marriage,
illiteracy Literacy is popularly understood as an ability to read and write Writing is a medium of human communication Communication (from Latin ''communicare'', meaning "to share") is the act of developing Semantics, meaning among Subject (ph ...
,
contraceptive Birth control, also known as contraception, anticonception, and fertility control, is a method or device used to prevent pregnancy Pregnancy, also known as gestation, is the time during which one or more offspring develops inside a woman ...
use, and reproductive compensation are the major determinants of apparent fertility, even amongst populations with a high proportion of consanguinous unions. However, several small effects on increased mortality, longer inter-birth intervals and reduced overall productivity have been noted in certain isolated populations.
Charles Darwin Charles Robert Darwin (; ; 12 February 1809 – 19 April 1882) was an English naturalist Natural history is a domain of inquiry involving organism In biology, an organism () is any organic, life, living system that fu ...

Charles Darwin
was one of the first scientists to demonstrate the effects of inbreeding depression, through numerous experiments on plants. Darwin's wife,
Emma Emma may refer to: * Emma (given name) Film * Emma (1932 film), ''Emma'' (1932 film), a comedy-drama film by Clarence Brown * Emma (1996 theatrical film), ''Emma'' (1996 theatrical film), a film starring Gwyneth Paltrow * Emma (1996 TV film), ''E ...

Emma
, was his first cousin, and he was concerned about the impact of inbreeding on his ten children, three of whom died at age ten or younger; three others had childless long-term marriages.


Factors reducing inbreeding depression

Whilst inbreeding depression has been found to occur in almost all sufficiently studied species, some taxa, most notably some angiosperms, appear to suffer lower fitness costs than others in inbred populations. Three mechanisms appear to be responsible for this: purging, differences in
ploidy Ploidy () is the number of complete sets of chromosome A chromosome is a long DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, wh ...
, and selection for
heterozygosity Zygosity (the noun, zygote A zygote (from Greek ζυγωτός ''zygōtos'' "joined" or "yoked", from ζυγοῦν ''zygoun'' "to join" or "to yoke") is a eukaryotic cell formed by a fertilization event between two gamete A gamete ( /ˈ ...
. It must be cautioned that some studies failing to show an absence of inbreeding depression in certain species can arise from small sample sizes or where the supposedly outbred control group is already suffering inbreeding depression, which frequently occurs in populations that have undergone a recent bottleneck, such as those of the
naked mole rat The naked mole-rat (''Heterocephalus glaber''), also known as the sand puppy, is a burrowing rodent native to parts of East Africa East Africa or Eastern Africa is the eastern subregion of the Africa Africa is the world's second-lar ...

naked mole rat
.


Purging selection

Purging selection occurs where the phenotypes of
deleterious{{Short pages monitor