There is a great deal of suspicion about consanguineous unions in the world. Whether this suspicion relies on health issues or not, we still have to be aware of possible genetic effects of consanguinity on heritable disorders and socio-cultural impacts.

Consanguinity itself does not make an individual "sick," but it affects the probability of an autosomal recessive disorder (which requires two copies of the same mutation) if there is such a mutation which runs within the family (with the word "if" emphasized). Consanguinity should be regarded as a complex issue due to socio-economic factors and health concerns.

In clinical genetics, unions between individuals who are second cousins or closer are considered "consanguineous" (inbreeding coefficient, F 0.0156). Incestuous unions (e.g. brother-sister, father-daughter) will not be included in the description of consanguinity in this article, and are obviously outlawed in every society and faith.

Speaking from a statistical geneticist's perspective, consanguinity affects the probabilities of certain genotypes occurring in offspring. For example, approximately 6.25% (F = 0.0625) of the alleles (a variation at a particular location in the genome) are expected to be homozygous in the offspring of first cousins; and 12.5% (F = 0.125) in the offspring of double first cousins. See Figure 1 for different types of consanguineous unions.

Historically speaking, consanguinity must have been unavoidable during the initial stages of mankind. However, even when human population sizes reached considerable figures, there is a great deal of evidence that consanguineous unions/marriages were widely practiced in ancient societies, especially amongst the ruling and/or land owning elite. For example, many Egyptian Pharaohs, Holy Roman Emperors, and members of the Spanish Habsburg dynasty used within-family unions as a way to keep the wealth, "royal blood," and/or "edification" within the family. 1-3 However, this is not to say taboos did not exist in other populations.

Though now taboo in Western countries, first cousin marriages were not uncommon in Europe and North America until the mid-19th century, again especially among individuals with high socio-economic status 7. For instance, Darwin married his first cousin, Emma (Wedgwood), in 1839. Yet, in the later half of the 19th century, scientific debates led to marriage between cousins being banned in 12 US states. 7,8

Consanguineous unions occur very rarely in Western countries for a variety of sociological (e.g. cultural) and statistical reasons, but the complete opposite is true in certain regions of the world where union of kin is seen as the default choice, for a variety of socio-economic reasons. Analysis of currently available data suggests that over one-tenth of the world's current population is composed of families where the parents are related as second cousins or closer 7.

Although a great majority of consanguineous families do not produce deficient offspring, it is still true that there is a risk of unions with close relatives.

The genetic risk comes from the fact that union of kin results in an increase of homozygosity (or more correctly autozygosity) in the offspring, which translates into a higher chance of being homozygous for any allele within the family gene pool. If any of these alleles are autosomal recessive mutations, this increase in probability applies to these alleles also. This simple fact is the main difference between union of kin and union of unrelated individuals (coined the name "inbreeding depression"); as there are many disease causal mutations out there in outbreeding populations (e.g. in Western populations), 21 but because they are very rare, they do not get to meet their counterpart in the union of unrelated individuals, thus their homozygous effects are never seen. This is why extremely rare autosomal recessive disorders are mostly seen in regions where consanguinity levels are high.

Despite the risk of heritable disease (albeit small, as discussed above), why are consanguineous marriages still common in many populations? There are many factors including socio-economic, ethno-cultural, religious and geographical; however the former duo (i.e. social and economic factors) seems to be key.17 Sociological studies in communities with high consanguinity rates show that the relationship between the husband and wife was found to be stable due to higher compatibility of life style, social relationships, and the family members from both sides. It is believed that consanguinity can also result in more female autonomy4 through better relationships with her in-laws as they can support her in times of need,17,22 Families concerned with the transmission of cultural values may also seem to practice it more. Consanguinity is a social reality of many communities. It relates to a number of social dynamics in respective cultural habitats with quite a number communal and health consequences. It is significant to be aware of these dynamics and consequences to be able to make a fair analysis on cultures and lifestyles unknown to us.

References

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19. Wikipedia.org. Cousin marriage law in the United States by state. Vol. 2013 (2013). http://en.wikipedia.org/wiki/Cousin_marriage_law_in_the_United_States_by_state

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27. Bittles, A. Consanguinity and its relevance to clinical genetics. Clin Genet 60, 89-98 (2001).

28. Dyer, O. MP is criticised for saying that marriage of first cousins is a health problem. BMJ 331, 1292 (2005).

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