What are the consequences of inbreeding?

What are the consequences of inbreeding?

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Inbreeding increases the risk of getting two identical recessive genes, alleles, that cause a disease which wouldn't have been activated with mixed genes. That's how I understand it anyway. But I sometimes read and hear that inbreeding among humans also causes decreased intelligence, especially emotional and social intelligence. Is there any support for such claims, and if so how does that work?

There is indeed evidence that inbreeding in humans lowers intelligence of offspring.

In summary, our comprehensive assessment revealed that parental consanguinity and degree of inbreeding was significantly associated with depression in intellectual behaviors among children. Factors other than inbreeding showed little influence, suggesting that genetic component (i.e., inbreeding) was more influential over these parameters under study. Moreover, the depression in cognitive abilities seems to be more prominent due to increase in the degree of inbreeding (F).

Estimating the Inbreeding Depression on Cognitive Behavior: A Population Based Study of Child Cohort

A thorough literature search resulted in 95 studies. There were 15 studies that provided sufficient data for a meta-analysis. The studies were divided into four categories of inbreeding, showing values of an inbreeding coefficient (f) of .046, .063 (first cousin marriage), .125 (double first cousin marriage), and .25 (incest). The study's hypothesis of a linear relationship was strongly confirmed. Over four categories, degree of inbreeding correlated highly with average IQ depression, r = .91. The current meta-analysis shows that the most common type of consanguinity, first cousin marriage (f = .063), may lead to a depression of about six IQ points, whereas incest (f = .25), the most severe type of consanguinity may cause a depression of about 28 IQ points.

Is there a linear relationship between inbreeding and mental ability?: A meta-analysis

Another study reaches a similar conclusion:

This result is interpreted in light of cultural feedback theory, whereby it is suggested that consanguinity could subtly influence IQ at larger scales as a result of small IQ handicaps bought about through inbreeding being amplified into much larger differences through their effect on factors that maximize IQ such as access to education and adequate nutrition.

About the mechanism behind the IQ decline caused by inbreeding:

The study of Morton (1978) study revealed that the offspring of first-cousins had over a five times higher risk of mental retardation when compared to controls. The study concluded that declines in IQ and the increase of mental retardation are consistent with rare recessive alleles associated with around 325 loci, whose likelihood of being transmitted into offspring increases with the relatedness of the parents.

Inbreeding depression and IQ in a study of 72 countries

You are right. Inbreeding strongly increases overall homozygosity which subjects inbred individuals to diseases caused by rare recessive alleles. In non-inbred individuals the chance is quite low to receive those because many deleterious variants (and in fact, most segregating alleles we can observe) are recessive. Most often, but depending on the dominance coefficient, these 'hide' in healthy heterozygous carriers and when very closely related individuals breed (with a lot of variants that are identical by descent) there is a high chance that some of the 'hidden' deleterious variants are passed to the offspring.

You have to be aware that there are in fact two conceptually different processes that can be both be referred to as inbreeding but are also somewhat continuously linked (see below):

  1. Mating systems that allow offspring between actually related individuals (measured by the degree of relatedness), what in humans often is referred to as incest or consanguinity.

  2. Populations with very low effective population size $N_e$ generally exhibit higher relatedness due to lack of genetic diversity (excess of homozygous sites). Therefore, populations with low $N_e$ are referred to as inbred even if no actually related individuals are mating.

There is some evidence that inbreeding in the first sense is linked to cognitive abilities in humans:

  • Bashi (1977) investigated the effect of offspring having first-cousin (i.e. between children of siblings) and double first-cousin (i.e. between children of siblings and unrelated siblings, sharing as much variants as half-siblings but with more recombination events) parents while (at least somewhat) correcting for socioeconomic effects. He found an inbreeding depression with respect to cognition that could

    result either from the general increase in homozygosity [… ] or from decrease in performance resulting from homozygosity for specific recessive alleles (highlighted by me). The higher variance of the double cousin group in some of the tests favours the second interpretation.

  • Woodley (2012) presents evidence for slightly lower IQs caused by inbreeding, however, he also mentions that the effect is way smaller than socioeconomic effects:

    Consanguinity could subtly influence IQ at larger scales as a result of small IQ handicaps bought about through inbreeding being amplified into much larger differences through their effect on factors that maximize IQ such as access to education and adequate nutrition.

  • Fareed and Afzal (2014) investigated verbal IQ, performance IQ, and full-scale IQ and found that all of these IQ parameters are significantly lower in inbred children compared to non-inbred children - actually the difference increases significantly with the degree of relatedness. They conclude that there is

    evidence for inbreeding depression on cognitive abilities among children.

Please keep in mind that, for for sociological/ethical reasons, this is highly controversial, especially when the two concepts above are intermixed - human populations underwent differential and variably strong periods of low $N_e$, i.e. were subject to stronger or weaker inbreeding in the second sense. When reading the citation of Bashi (1977) above carefully, you will notice that he takes good care not to intermix those. Inbreeding in the first sense leaves large runs of homozygosity (ROH, blocks without heterozygous sites, i.e. a clustered and local lack of variation) whereas inbreeding in the second sense increases homozygosity in the genome less selectively. Therefore, the distribution of homozygous sites can be used to infer whether inbreeding is recent (first sense) or old (second sense) (see for example McQuillan et al. (2008) - here you see that both concepts form a continuum: where do you set the cutoff between recent and old? what is an appropriate threshold size for ROH?… ). Regardless, Bashi's findings indicate that effects of inbreeding are caused by recent inbreeding as he presents evidence that his results are rather driven by specific deleterious recessive variants than overall excess homozygosity. Finally, even though the study by Fareed and Afzal (2014) shows rather large effects of recent inbreeding in IQ measures, the results from Woodley (2012) show that one needs to be really careful to separate genetic from environmental components as his study suggests that the latter contribute more to the observed decrease in IQ.

Inbreeding, in nature at large, has one primary effect, as you said. It increases the chances of two copies of harmful, recessive alleles. Consequently, offspring are much more likely to suffer from genetic and/or degenerative diseases.

As for intelligence, some studies have indeed concluded that inbreeding can lead to a lower IQ (see this answer for the exact quotations), it is possible that there are other factors at play. For instance, those from a lower socio-economic background tend to be of lower intelligence, although as with everything there are exceptions. Whilst I have struggled to find a source for this, it is statistically more likely that those of a lower socio-economic background will exhibit poor mental health and/or partake in the act of inbreeding.

Consequently, due to the fact that intelligence is largely heriditary, the offspring of inbreeding will tend to be less intelligent. As a result, it is difficult to determine that there is a causal relationship between these correlating factors.

My understanding, NOT based on any study, it purifies the gene 'pool' since mutation derived alleles variation will be lost thru' generations just like we don't have much of our grand grand's 'unique' allele.

This is bad as it happens in farms, when an decease that infects one, could eliminates all the populations, to be exact, barring a few left.

In the ever changing environment as in evolutions, singular gene pool will be hit with extinction even thought it might prosper if and only if the same good old 'environment' persists. Environment here includes everything in the ecosystems.

Genetics and broodstock management of coho salmon

1.1.2 Inbreeding

Inbreeding is defined as the probability of two alleles in an individual being identical by descent, and is normally the result of mating related individuals. The rate of inbreeding is a function of the characteristics of the foundation stock as well as limited population sizes in subsequent generations ( Falconer, 1989 ). The deleterious effects of inbreeding have been documented in aquatic species, but there has been particular emphasis on salmonids ( Aulstad and Kittelsen, 1971 Kincaid, 1976, 1983, 1995 Gjerde et al., 1983 Su et al., 1996 ). The majority of these studies produced relatively high inbreeding levels (∆F = 10–25%) through sib matings. Hershberger et al. (1990a) analyzed the growth performance of coho salmon under selection and increasing levels of inbreeding. Despite accumulated inbreeding levels after four generations approaching those of full-sibmating, there was no apparent decrease in growth performance. Whether selection gains masked deleterious effects or the accumulation of inbreeding levels over several generations does not result in the same deleterious effects as has been reported for closely related (sib) matings was not determined.

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General Overviews

Classical population genetics has long concerned itself with the systems of mating and population structures that generate inbreeding and inbreeding depression, the decline in fitness due to inbreeding (see Early-20th-Century History of Inbreeding Research and Textbooks). Population geneticists exploring the forces that generate and maintain genetic variation analyze how the steady flow of deleterious mutations is countered by selection resulting in an equilibrium genetic load (mutation-selection balance). Recessive deleterious alleles are exposed as homozygotes upon inbreeding, expressing the load as inbreeding depression. Charlesworth and Charlesworth 1987, Crow 1993, and Charlesworth and Charlesworth 1999 provide authoritative reviews of the origin and dynamics of the inbreeding load. Charlesworth and Willis 2009 emphasizes the standing genetic variation in fitness that exists in most populations and summarizes evidence that most inbreeding depression reflects the segregation of dominant rather than overdominant alleles. The evolutionary dynamics of these load loci can be complex and depends on many factors including levels of dominance, fitness effects, how these vary over environments, epistatic interactions among loci, gene flow, and the associations that arise among loci. The authors mentioned above, Keller and Waller 2002, and Hedrick and Garcia-Dorado 2016 all emphasize how these complex effects make it difficult to predict dynamics of the inbreeding load. The volume edited by Thornhill 1993 (see Textbooks) presents a diversity of perspectives on inbreeding in a wide range of organisms with authors ranging from theoreticians to experts in particular groups, including the creative evolutionist W. D. Hamilton. Inbreeding is both a tool and a hazard in plant and animal breeding, leading Kristensen and Sørensen 2005 to review the theory and empirical results on inbreeding effects so they can recommend how to minimize its risks in breeding programs. Inbreeding depression is also a major concern in conservation biology, a topic reviewed by Hedrick and Kalinowski 2000. New methods and the ready availability of DNA sequence data in recent years (see Methods to Measure Inbreeding) are now providing more detailed pictures of inbreeding and its evolutionary consequences in wild populations.

Charlesworth, D., and B. Charlesworth. 1987. Inbreeding depression and its evolutionary consequences. Annual Review of Ecology and Systematics 18:237–268.

Review of how the genetic load of mostly recessive deleterious mutations manifests as inbreeding depression and how these effects modify load dynamics and the evolution of other (e.g., reproductive) characteristics. Reviews support for the role of dominance rather than overdominance as the genetic basis for inbreeding effects.

Charlesworth, B., and D. Charlesworth. 1999. The genetic basis of inbreeding depression. Genetical Research 74:329–340.

A clear review of the theory and empirical evidence for directional dominance as the primary mechanism causing inbreeding depression. They also describe the meager data available on the distribution of mutational effects and implications of these for how inbreeding depression evolves.

Charlesworth, D., and J. H. Willis. 2009. The genetics of inbreeding depression. Nature Reviews Genetics 10:783–796.

A general review of the basis for inbreeding depression, drawing on both theory and recent empirical evidence. The article stresses the practical significance of inbreeding depression for understanding the dynamics of mating system evolution and methods and outcomes in plant and animal breeding.

Crow, J. F. 1993. Mutation, mean fitness, and genetic load. Oxford Series in Evolutionary Biology 9:3–42.

Authoritative historical review of the ideas and theory related to genetic load, including how recurrent mutations with various effect sizes and levels of dominance affect population fitness. Emphasizes how the architecture of the genetic load varies depending on population size and discusses evidence that dominance rather than overdominance tends to generate most of the inbreeding depression we observe.

Hedrick, P. W., and S. T. Kalinowski. 2000. Inbreeding depression in conservation biology. Annual Review of Ecology and Systematics 31:139–162.

A review of the evidence for inbreeding depression, purging, and genetic rescue in the context of the management and the conservation of endangered species.

Hedrick, P. W., and A. Garcia-Dorado. 2016. Understanding inbreeding depression, purging, and genetic rescue. Trends in Ecology & Evolution 31:940–952.

An age-of-genomics review of how mutation, gene flow, and selection interact to affect dynamics of the inbreeding load. Both selection on deleterious alleles (purging) and the introduction of beneficial alleles through gene flow at loci with a high frequency of deleterious alleles (genetic rescue) can act to reduce the inbreeding load and are therefore important for the survival of inbred populations. However, such benefits also involve risks that may limit their success.

Keller, L. F., and D. M. Waller. 2002. Inbreeding effects in the wild. Trends in Ecology & Evolution 17:230–241.

A broad overview that reviews basic theory as well as growing empirical evidence for the significance of inbreeding effects under natural conditions. Highlights key studies confirming that effects of inbreeding and fixation can be rapidly reversed when organisms from small isolated populations are crossed with those from large populations (genetic rescue).

Kristensen, T. N., and A. C. Sørensen. 2005. Inbreeding—lessons from animal breeding, evolutionary biology and conservation genetics. Animal Science 80:121–133.

Animal breeders inbreed livestock to expose traits and select for economic traits. Kristensen and Sørensen review theory and empirical results on model organisms to conclude that inbreeding depresses both mean fitness and quantitative genetic variation. Because these effects could limit breeding success, the authors stress the importance of controlling inbreeding in livestock populations.

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Local effects of inbreeding on embryo number and consequences for genetic diversity in Kerguelen mouflon

A classical paradigm in population genetics is that homozygosity or inbreeding affects individual fitness through increased disease susceptibility and mortality, and diminished breeding success. Using data from an insular population of mouflon (Ovis aries) founded by a single pair of individuals, we compare embryo number of ewes with different levels of inbreeding. Contrary to expectations, ewes with the highest levels of homozygosity showed the largest number of embryos. Using two different statistical approaches, we showed that this relationship is probably caused by heterozygosity at specific genes. The genetics of embryo number coupled with cyclic dynamics could play a central role in promoting genetic variation in this population.


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Genetics of consanguinity and inbreeding in health and disease

Context: Inbreeding increases the level of homozygotes for autosomal recessive disorders and is the major objective in clinical studies. The prevalence of consanguinity and the degree of inbreeding vary from one population to another depending on ethnicity, religion, culture and geography. Global epidemiological studies have revealed that consanguineous unions have been significantly associated with increased susceptibility to various forms of inherited diseases.

Objective: The study aimed to determine the role of consanguinity in human health and to highlight the associated risks for various diseases or disorders.

Methods: PubMed and Google Scholar search engines were used to explore the published literature on consanguinity and its associated risks using the key words "consanguinity", "prevalence", "inbreeding depression", "coefficient of inbreeding", "child health", "mortality", "human health", "homozygosity" and "complex diseases" in different combinations. The studies were screened for eligibility on the basis of their epidemiological relevance.

Results: This comprehensive assessment highlights the deleterious consequences in populations with a higher prevalence of consanguinity among different countries worldwide.

Conclusions: To avoid the inbreeding load there is the need to improve socioeconomic and educational status and to increase public awareness of reproductive health and anticipated deleterious effects. Pre-marital and pre-conception counselling of consanguineous populations should be an integral part of health policy to train people and make people aware of its harmful consequences. Furthermore, runs of homozygosity (ROH) and whole-exome sequencing (WES) are useful tools in exploring new genomic signatures for the cause of inbreeding depression.

Keywords: Consanguinity inbreeding public health runs of homozygosity.

Genome sequencing reveals extensive inbreeding in Scandinavian wolves

Credit: CC0 Public Domain

Researchers from Uppsala University and others have for the first time determined the full genetic consequences of intense inbreeding in a threatened species. The large-scale genomic study of the Scandinavian wolf population is reported in Nature Ecology & Evolution.

The Scandinavian wolf population was founded in the 1980s by only two individuals. This has subsequently led to intense inbreeding, which is considered a long-term threat to the population. To reveal the genetic consequences of inbreeding, the whole genome of some 100 Scandinavian wolves has now been analysed.

'Inbreeding has been so extensive that some individuals have entire chromosomes that completely lack genetic variation', says Hans Ellegren, Professor at the Evolutionary Biology Centre, Uppsala University and leader of the study. 'In such cases identical chromosome copies have been inherited from both parents.'

A surprising discovery was that also some immigrant wolves were partly inbred, and related. This was the case, for example, for two wolves that 2013 were translocated by management authorities from northernmost Sweden, due to conflict with reindeer husbandry, to southern Sweden. This is counter to the often-made assumption of unrelated and non-inbred founders when inbreeding is estimated from pedigrees.

'The degree of inbreeding determined at high precision with genome analysis agreed rather well with inbreeding estimated from established pedigrees', says Hans Ellegren. 'However, for stochastic reasons, some wolves were found to be a bit more, and others a bit less, inbred than estimated from pedigrees.'

Moreover, wolves were generally more inbred than expected from recent mating between relatives in the contemporary population. This is because the two copies of a chromosome in an individual can originate from one and the same ancestor further back in time.

Beetles To Be Used To Show Consequences Of Inbreeding

They are cursed the world over for contaminating food supplies and are a huge commercial pest, but the humble flour beetle is about to play a significant role in the management of endangered species.

The flour beetle &ndash or Tribolium castaneum &ndash will be the model in a major new study into the consequences of inbreeding, launched at the University of East Anglia (UEA) this summer.

Inbreeding is a potentially important problem in declining species across the world, and conserving genetic variation is now recognised as a priority by the International Union for Conservation of Nature.

When populations become isolated or depleted due to loss of habitat or exploitation, the gene pool is reduced which forces inbreeding between relatives, and therefore losses in genetic variability. This results in the increased expression of &lsquobad genes&rsquo which were previously hidden in a variable genome, leading to a range of problems usually associated with reproduction. This inbreeding depression then adds to the habitat loss or exploitation problems, further driving a population towards extinction.

Funded by the Natural Environment Research Council (NERC), the £400,000 project will experimentally evaluate which specific reproductive traits are affected by inbreeding, and how they impact on population viability. Once inbreeding depression and its causes are discovered, the project will determine how much new variability must be re-introduced to genetically rescue an inbred population from extinction.

The results of the three-year study will help managers of conservation and captive breeding projects recognise when inbreeding is a problem, how it progresses, and how best to manage or reverse it.

&ldquoWhen a species is under threat, it is difficult to be sure whether inbreeding or other stresses are responsible for declines in numbers,&rdquo said Dr Matthew Gage of UEA&rsquos School of Biological Sciences.

&ldquoOur previous work on wild rabbits in the natural environment implicates sperm as being sensitive to inbreeding. However, these large-scale experimental trials in the laboratory will shed new light on the relative importance of inbreeding as a conservation concern, identify which traits are most sensitive, and therefore could help us recognise populations under threat of inbreeding and manage their recovery.&rdquo

Story Source:

Materials provided by University of East Anglia. Note: Content may be edited for style and length.

What impact has recent inbreeding had on the genetic diversity of domestic dogs?

Research published today in Canine Genetics and Epidemiology investigated the rate of inbreeding in domestic dog breeds recognized by The Kennel Club. Here, Tom Lewis explains more.

The term ‘inbreeding’ is widely associated with severe physiological (and in humans, mental) impairment as a consequence of parents being closely related. As a result, incestuous relationships are a widespread cultural taboo, with laws against incestuous marriages, at least of first degree relatives, almost universally present throughout the world.

However, it is actually very difficult to avoid any degree of relationship between two parents. In part this is due to simple mathematics – the number of ancestors increases exponentially with each generation further back (two parents, four grand-parents, eight great-grand-parents) meaning that it is inevitable there will be some degree of common ancestry if we go back far enough.

Less well understood however, and particularly important for domestic animal species, is the intrinsic relationship between inbreeding and selection.

Less well understood however, and particularly important for domestic animal species, is the intrinsic relationship between inbreeding and selection.

That relatives resemble each other is a central tenet of genetics. Therefore, if we select breeding animals that resemble each other with respect to a particular trait, then these individuals will on average be more closely related than if mating was random.

This means that selection will lead to some degree of inbreeding. Consequently, rather than attempting to avoid any inbreeding in domestic animal populations at all, a more useful strategy is to manage the loss of genetic diversity (rate of inbreeding) to within sustainable levels.

Calculating the rate of inbreeding

The domestic dog shows a greater variety in appearance and behavior than any other domestic species. Many distinct breeds were formed and are maintained by closed registries which have led to the widespread belief that pedigree dogs are very inbred.

Crucially however, the rigorous recording of pedigree data provides large datasets from which genetic parameters and historical trends can be determined, and this information used to guide future breeding strategies.

The rates of inbreeding calculated in all 215 domestic dog breeds recognized by the Kennel Club over the period 1980 to 2014 form the results central to a study recently published in Canine Genetics and Epidemiology. Although the precise profile of rate of loss of genetic diversity is unique for each breed, the study did determine some broad trends.

What did we find?

There was a general contraction of within-breed genetic diversity in the 1980s and 1990s, with the overall rate of inbreeding at a level at which detrimental effects, such as inbreeding depression, would be expected to be observed. However, since the turn of the century the rate of inbreeding has tended to decline across breeds, implying breeders have taken steps to conserve genetic diversity.

Interestingly this general decline in the rate of inbreeding coincides with the relaxation of the UK’s quarantine laws. This suggests that breeders may have taken the opportunity provided by new laws easing travel restrictions for dogs to slow the erosion of genetic diversity, or even re-introduce some genetic variation, through more widespread use of non-UK animals in breeding.

Perhaps surprisingly there appeared to be no relationship between the rate of inbreeding observed and population size.

Perhaps surprisingly there appeared to be no relationship between the rate of inbreeding observed and population size (as judged by mean number of Kennel Club registrations). This implies that it is possible to conserve genetic diversity via a sustainable rate of inbreeding in small populations, perhaps through judicious use of migrant animals for breeding to provide an injection of genetic diversity to the population.

The existence of popular sires, or male parents, was observed in virtually all pedigree dog breeds in the study. The repeated use of prolific breeding males is both often a feature of selection and a contributor to a high rate of inbreeding, and is typical in breeding practices of almost all domestic mammal species.

Intense selection in males

The much greater reproductive capacity of males compared to females means more intense selection can be applied to males, and the few selected then make a far greater individual genetic contribution to future generations.

While this results in widespread dissemination of genes influencing ‘sought after’ traits (the response to selection), it also makes such males much more likely to be a common ancestor to all breeding animals in subsequent generations, increasing the rate of inbreeding.

The challenge remains, for the dog as for other domestic species, to achieve sufficient response to selection (for traits related to health, temperament, and so on) at a rate of loss of genetic diversity which is sustainable.

How to avoid a high level of inbreeding within a gene pool?

Outbreeding is bringing in a member from a different gene pool to breed with a female and vary the gene pool, increasing the gene pool’s health. With breeding, it is easy to hire an AKC approved stud dog for breeding to allow for a healthy and branched litter of pups. The benefit of this is that you can look through a stud catalog to find a male that has your desired traits, while also not minimizing the health of the litter produced. Similarly, you can pay for a female to be fertilized by your stud dog and then gain ownership of the litter produced. This means the members of the litter you keep for breeding are genetically more healthy.

Heterosis is another suggested method of creating a healthier gene pool, defined as cross-breed breeding. This remains a controversial subject in the dog breeding industry due to implied health risks/ argued health benefits and influence on physical appearance. Many scientific minds state that this can be hugely beneficial due to the introduction of new genetics into the breed also known as hybrid vigor. Many experts debate this is merely a myth though and that outbreeding depression can cause reduced health in the offspring.

If breeders are determined to inbreed, it is strongly recommended to breed individuals who are distantly related and to monitor the coefficient of inbreeding regularly. Distantly related means the parents should not be parent and child, siblings or even first siblings. Ideally, though, outbreeding needs to take place to introduce a new genetic pool and maintain a high level of health within the family and for the offspring. Ultimately, in-breeding does have a proven negative impact and breeders have a responsibility to maintain high levels of health for all their animals.


  1. Agenor

    You must say this - the big fault.

  2. Kigul

    no, I don't like that!

  3. Rosco

    YES, the variant is good

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