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Offspring of biologically related persons are subject to the possible effect of inbreeding, such as congenital birth defects. The chances of such disorders is increased the closer the relationship of the biological parents. (See coefficient of inbreeding.) This is because such pairings increase the proportion of homozygous zygotes in the offspring, in particular deleterious recessive alleles, which produce such disorders.[8] (See inbreeding depression.) Because most recessive alleles are rare in populations, it is unlikely that two unrelated marriage partners will both be carriers of the alleles. 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. 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 population. In the short term, incestuous reproduction is expected to produce increases in spontaneous abortions of zygotes, perinatal deaths, and postnatal offspring with birth defects.[9] 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.[10]

Malformations or harmful traits can stay within a population due to a high homozygosity rate and it will cause a population to become fixed for certain traits, like having too many bones in an area, like the vertebral column in wolves on Isle Royale or having cranial abnormalities in Northern elephant seals, where their cranial bone length in the lower mandibular tooth row has changed. Having a high homozygosity rate is bad 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.[11] When there are deleterious recessive alleles in a population it can cause inbreeding depression. The authors think that it is possible that the severity of inbreeding depression can be diminished if natural selection can purge such alleles from populations during inbreeding.[12] If inbreeding depression can be diminished by natural selection than some traits, harmful or not, can be reduced and change the future outlook on a small, endangered populations.


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 inbreeding depression becoming less severe. This is associated with the unmasking and eliminating of severely deleterious recessive alleles. It is not likely, though, that eliminating can be so complete that inbreeding depression is only a temporary phenomenon. 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 genomes accumulate deleterious mutations that are irreversible.[14]

Genetic disorders[edit]

Autosomal recessive disorders occur in individuals who have two copies of the gene for a particular recessive genetic mutation.[15] 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 gene, and therefore their children are at a higher risk of a 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 general population.[16] A study has provided the evidence for inbreeding depression on cognitive abilities among children, with high frequency of developmental disability among offspring in proportion to their increasing inbreeding coefficients.[17]

Children of parent-child or sibling-sibling unions are at increased risk compared to cousin-cousin unions.[18]

Inbreeding may result in a far higher phenotypic expression of deleterious recessive genes within a population than would normally be expected.[19] As a result, first-generation inbred individuals are more likely to show physical and health defects[citation needed], including:

Inbreeding can occur just because a small population has been isolated during some time, so that all breeding individuals became genetically related. It can also occur in a large population if individuals tend to mate with their relatives, instead of mating at random.













Measures of inbreeding[edit]

A measure of inbreeding of an individual A is the probability F(A) that both alleles in one locus are derived from the same gene in an ancestor. Two alleles derived from the same gene in an ancestor are said to be identical by descent. This probability F(A) is called the "coefficient of inbreeding".[30]

Another useful measure that describes the extent to which two individuals are relatives (say individuals A and B) is their coancestry coefficient f(A,B), which gives the probability that, taking one random allele from A and another random allele from B, both are identical by descent. This is also denoted kinship coefficient between A and B.




Typical coancestries between relatives are as follows:

  • First cousins → 6.25% (116)

Domestic animals[edit]



Breeders must cull unfit breeding suppressed individuals or individuals who demonstrate either homozygosity or heterozygosity for genetic based diseases.[35] The issue of casual breeders who inbreed irresponsibly is discussed in the following quotation on cattle:







Laboratory animals[edit]



Effects of Inbreeding in Humans[edit]

Consanguineous relations have been tabooed for centuries and for good reason. The partners in these relationships that inbreed, simultaneously jeopardize their offspring's fitness and bring upon them great evolutionary risk. Inbreeding opens the door for prevalence of deleterious recessive alleles by increasing their chance for homozygosity and thereby, impeding the fitness of the offspring. With continuous inbreeding, the loss of genetic variation magnifies, and in turn, enables the expression of such recessive alleles following boundless negative implications.[41] When determining the degree of inbreeding in an individual, scientists use a term called the inbreeding coefficient which estimates the percent of homozygous alleles in the overall genome.[42] The more biologically related the inbreeders are, the greater the inbreeding coefficient (See Coefficient of Inbreeding), being that 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.[43] 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.[43]

Inbreeding is especially dangerous in small populations where the genetic variation is already limited.[44] By inbreeding, individuals are further limiting the variation of alleles and ensure a higher percentage of homozygosity of their offspring's genome.[45] Thus, the likelihood of deleterious recessive alleles to pair is significantly higher in an small inbreeding population than in a larger inbreeding population.[44] By consistently increasing the homozygosity of alleles, the population is, in turn, allowing these harmful alleles to dominate, leading to expression of genetic disorders.[44] Due to the inflicted individuals decreased fitness and reproductive success, the recessive alleles will eventually become culled by natural selection.[46] Thus in small inbreeding populations, deleterious recessive alleles are more likely to be prevalent short term, but will decrease more swiftly long term after consistent expression followed by natural selection.[46]

The fitness consequences of consanguineous mating have been studied since their scientific recognition by Charles Darwin in 1839.[47][48] Some of the most harmful effects known from such breeding include its effects on the mortality rate, as well as the general health of the offspring.[49] Within the past several decades, there have been many studies to support such debilitating effects on the human organism.[41][45][47][49][50][51] Specifically, inbreeding has been found to decrease fertility as a direct result of increasing homozygosity of deleterious recessive alleles.[51][52] Fetuses produced by inbreeding also face a greater risk of spontaneous abortions due to inherent complications in development.[53] 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.[54] Additionally, consanguineous parents possess a high risk of premature birth and producing underweight and undersized infants.[55] Viable inbred offspring are also likely to be inflicted with physical deformities and genetically inherited diseases.[42] 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.[42][56] 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.[57] A study in 2009 revealed that inbred individuals have a greater susceptibility to TB and hepatitis.[58] Being that resistance to these diseases is dependent on one's immune system, it is thereby suggested that consanguinity increases individual's susceptibility to infectious diseases.[58] While inbreeding may seem to mainly implicate physical disparities, it also strongly affects cognitive ability. A recent behavioral study discovered that parental consanguinity followed by inbreeding is significantly correlated with the depression in intelligence among children.[41] Furthermore, such offspring are known to have a lower IQ and higher risk for developmental disabilities. [50]

Prevalence of Inbreeding[edit]

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.[44] Of the practicing regions, Middle Eastern and northern Africa territories show the greatest frequencies of consanguinity.[44] The link between the high frequency and the region is primarily due to the dominance of Islamic populations, who have historically engaged in familyline relations.[48]

  • Afghanistan & Lebanon- Populations within Afghanistan and Lebanon have also showed elevated levels of consanguinity. Studies determined that in Afghanistan, inbreeding was more heavily practiced in the more rural, western and southern regions.[61] In Lebanon, a study focused on the dominating Shi'a populations and ascertained that about 28% of the marriages were consanguineous.[62]
  • Israel- The Bedouin population in Israel has commonly practiced consanguinity throughout their existence, with studies illustrating its prevalence in modern day still. In recent years, of the Bedouin women studied, 45% of the women have been found to be in a consanguineous relationship of sort, with first cousin marriages the most common. The level of education and age of marriage were found to be significantly correlated with consanguinity, with lower levels of education and younger age of marriage influencing higher bloodline marriage rates. Evidently, efforts should be made to better educate the population on the repercussions of inbreeding in hopes to decrease their risk of congenital malformations and genetic diseases.[63] While rates of inbreeding may still be high, those of Israeli Arabs have been decreasing. A 2010 study analyzed the rate of consanguineous marriages among Israeli Arabs from 2000-2009 and ascertained a steady decline from roughly 36% to 24%.[64] Similar to the Bedouin population, most of the women who engaged in consanguinity married at a younger age, thus premarital counseling is highly advised to prevent such statistics.


  • Tunisia- Specifically, studies in 2006 found that among a Tunisian population examined, about 20% of the individuals were inbred.[65] There was also a consistent correlation of increased frequency of consanguineous relations with a lower education level and occupation status.
  • Egypt- In Egypt, researchers have observed high levels of inbreeding among populations, with consanguinity frequencies as high as 38% in rural regions. Moreover, a study done in 2010 concluded a link between bipolar I disorder and Schizophrenia, two genetic disorders prevalent in Egypt, and consanguinity.[66]

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.[44] Furthermore, among inbred children in Palestine and Lebanon, there is a positive association between consanguinity and reported cleft lip/palate cases.[44] For further information on inbreeding effects, explore Tadmouri's review, "Consanguinity and reproductive health among Arabs," Bittle's article, "Consanguinity, human evolution, and complex diseases," and Hamamy's review, "Consanguineous marriages."[citation needed]

Possible increase of fertility[edit]


Royalty and nobility[edit]

Main article: Royal intermarriage



  • Among European monarchies, Jean V of Armagnac formed a rare brother-sister relationship, as well as Ferdinand I of Portugal with his half-sister. Also, other royal houses, such as the Wittelsbachs, had marriages among aunts, uncles, nieces and nephews.
  • Another famous genetic disease that circulated among European royalty was hemophilia. This spread to the royal families of Russia and Spain, and was a factor in the overthrow of both. Because the progenitor, Queen Victoria, was in a first cousin marriage, it is often believed that the cause was consanguinity (inbreeding). However, this disease (in males) is generally not aggravated by cousin marriages, although rare cases of hemophilia in girls can occur from the union of hemophiliac men and their cousins.[75][76]


See also[edit]


  1. ^ Inbreeding at Encyclopædia Britannica
  2. ^ Nabulsi MM, Tamim H, Sabbagh M, Obeid MY, Yunis KA, Bitar FF (2003). "Parental consanguinity and congenital heart malformations in a developing country". American journal of medical genetics. Part A 116A (4): 342–7. doi:10.1002/ajmg.a.10020. PMID 12522788. 
  4. ^ Chen X. (1993). "Comparison of inbreeding and outbreeding in hermaphroditic Arianta arbustorum (L.) (land snail)". Heredity 71 (5): 456. doi:10.1038/hdy.1993.163. 
  18. ^ Wolf, Arthur P. and Durham, William H., ed. (2005). Inbreeding, incest, and the incest taboo: the state of knowledge at the turn. Stanford University Press. p. 3. ISBN 0804751412. 
  20. ^ Fareed, M; Afzal, M (2014). "Evidence of inbreeding depression on height, weight, and body mass index: A population-based child cohort study". Am J Hum Biol. 26 (6): 784–795. doi:10.1002/ajhb.22599. PMID 25130378. 
  22. ^ Kirkpatrick M, Jarne P (2000). "The Effects of a Bottleneck on Inbreeding Depression and the Genetic Load". The American naturalist 155 (2): 154–167. doi:10.1086/303312. PMID 10686158. 
  30. ^ Wright, S (1922). "Coefficients of inbreeding and relationship". Amer. Natur 56: 330–338. doi:10.1086/279872. 
  34. ^ a b c Tave, Douglas and (1999). Inbreeding and brood stock management. Food and Agriculture Organization of the United Nations. p. 50. ISBN 978-92-5-104340-0. 
  41. ^ a b c Fareed, Mohd; Afzal, Mohammad (2014-10-14). "Estimating the Inbreeding Depression on Cognitive Behavior: A Population Based Study of Child Cohort". PLoS ONE 9 (10): e109585. doi:10.1371/journal.pone.0109585. PMC 4196914. PMID 25313490. 
  43. ^ a b Kamin, Leon J. (1980-01-01). "Inbreeding depression and IQ.". Psychological Bulletin 87 (3). doi:10.1037/0033-2909.87.3.469. 
  47. ^ a b . doi:10.1017/S0140525X00014850.  Missing or empty |title= (help)
  49. ^ a b Ober, Carole; Hyslop, Terry; Hauck, Walter W. (1999-01-01). "Inbreeding Effects on Fertility in Humans: Evidence for Reproductive Compensation". The American Journal of Human Genetics 64 (1). doi:10.1086/302198. PMC 1377721. PMID 9915962. 
  52. ^ Ober, C., Elias, S., Kostyu, D. D., & Hauck, W. W. (1992). Decreased fecundability in Hutterite couples sharing HLA-DR. American Journal of Human Genetics, 50(1), 6–14.
  53. ^ Diamond, J. M. (1987). Causes of death before birth. Nature, 329(6139), 487–488.
  54. ^ Stoltenberg, C., Magnus, P., Skrondal, A., & Lie, R. T. (1999). Consanguinity and recurrence risk of stillbirth and infant death. American Journal of Public Health, 89(4), 517–523.
  55. ^ Khlat, M. (1989). Inbreeding effects on fetal growth in Beirut, Lebanon. American Journal of Physical Anthropology, 80, 481−484.
  56. ^ Bener, A., Dafeeah, E. E., & Samson, N. (2012). Does consanguinity increase the risk of schizophrenia? Study based on primary health care centre visits. Mental Health in Family Medicine, 9(4), 241–248.
  58. ^ a b Template:Lyons, E.J., Frodshm, A.J., Zhang, J., Hill, A. V.S. & Amos, W. (2009). Consanguinity and susceptibility to infectious diseases in humans. Biology Letters, 5, 574-6.
  61. ^ Saadat, M., & Tajbakhsh, K. (2013). Prevalence of consanguineous marriages in west and south of Afghanistan. Journal of Biosocial Science, 45(6), 799–805.
  62. ^ El-Kheshen, G., & Saadat, M. (2013). Prevalence of consanguineous marriages among shi’a populations of Lebanon. Journal of Biosocial Science, 45(5), 675–82.
  71. ^ Bevan, E.R. "The House of Ptolomey". 
  73. ^ "The Imperial House of Habsburg: Chapter 5. Web page accessed September 23, 2007". Archived from the original on August 27, 2007. 
  75. ^ Lock, Stephen; Last, John M. and Dunea, George (2001). The Oxford illustrated companion to medicine. Oxford University Press US. p. 329. ISBN 978-0-19-262950-0. 
  76. ^ Bainbridge, David (2004). The X in Sex: How the X Chromosome Controls Our Lives. Harvard University Press. p. 88. ISBN 978-0-674-01621-7. 

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