Consanguineous marriage: facts versus fiction

Author: A. Bittles
Submitted: Friday 6th of July 2012 09:44:48 AM
Submitted by: egf
Language: English
Content type: Learning resource
Educational levels: expert, qc2, qc3

Abstract

Close kin relationships have a long history in human populations, as evidenced in modern Western societies by the genomic evidence of extensive runs of homozygosity (ROH) resulting from consanguineous unions in much earlier generations. Estimates of the numbers of mature adults who took part in the original Out-of-Africa migration approximately 70,000 years ago range from just 700 to 10,000 individuals, and since the migration involved small groups of people following a hunter-gatherer existence close kin mating was inevitable. Even after the onset of settled agriculture and the formation of city states around 3,000 BC, increasing social stratification meant that people continued mainly to reproduce within numerically small and enclosed kinships and sub-communities. This tradition of community endogamy continues in many of the more populous regions of the world, with marriage within clans, tribes, castes and biraderi strongly favoured, and in many cases obligatory. From a medical genetics perspective consanguineous marriage, a term derived from the Latin con sanguineus (of the same blood), is conventionally defined as a union between two individuals related as second cousins or closer. In consanguineous unions the partners share genes inherited from one or more common ancestor and, for example, in first cousin marriages the spouses are predicted to have 12.5 percent of their genes in common. This means that on average their progeny will be homozygous at 6.25 percent of gene loci, i.e. they will have received identical gene copies from each parent at these sites in their genome. If the same mutant gene is inherited from both parents the individual will express the disorder, either at birth or later in life depending on the nature and site of the mutation, thus contributing to the phenomenon of inbreeding depression. The coefficient of inbreeding (F) is a numerical estimate of the degree of inbreeding of an individual, and so for first cousin offspring F = 0.0625. Similarly, for the progeny of uncle-niece unions F = 0.125, while for second cousin offspring F = 0.0156. In many communities there is a long, unbroken history of consanguineous unions and the cumulative level of inbreeding may be significantly higher than the value calculated for a single generation. Under these circumstances, where a detailed, multi-generational pedigree is available a correction can be applied to account for the effects of ancestral inbreeding using the formula: F = Σ (1/2)n (1+FA) where FA is the ancestor’s inbreeding coefficient, n is the number of individuals in the path connecting the parents of the individual, and the summation (Σ) is taken over each path in the pedigree that goes through a common ancestor. Although consanguinity is now rare in most Western societies and is largely restricted to non-European migrant communities, marriage between close biological relatives remains favoured in many parts of Asia 8 and Africa, and to a lesser extent in South America, with at least 1,100 million people either married to spouses who are second cousins or closer or the progeny of such unions. The precise types of consanguineous marriage that are contracted differ by geographical region and are strongly influenced by factors such as religion, cultural traditions and socioeconomic status. Meta-analyses based on 75 study populations resident across four continents with a total sample size of ~5 million individuals have shown a positive association between consanguinity and fertility that may in part reflect reproductive compensation. Comparing the outcomes of first cousin and non-consanguineous pregnancies there was a mean consanguinity-associated excess of 5/1,000 stillbirths, 12.5/1,000 infant deaths, and 34/1,000 deaths from approximately 28 weeks gestation to 10/12 years of age. However, many of the studies included in these analyses had poor control for non-genetic variables, including maternal age, birth interval, residence, and socioeconomic status, meaning that the data should best be regarded as upper level estimates. Similar problem are apparent with studies on birth defects, including congenital heart defects, neural tube defects and oral and facial clefts, with different surveys reporting highly variable results. There also has been a general failure to account for community endogamy, often described in terms of population stratification, which can lead to significant misinterpretation of study results and the reporting of exaggerated consanguinity-associated disease risks. During the last decade consanguinity has become a contentious subject in many European countries, and in the USA where first cousin marriage is restricted or prohibited in 31 of the 50 states. Periodic calls for a ban on marriages between first cousins largely are based on questionable data, which recent re-assessment has shown to be incomplete and often significantly biased. There is an urgent need for new studies to resolve this important issue, incorporating genomic analysis and with appropriate attention paid to the accompanying demographic, social and economic parameters. Comprehensive investigations, such as the planned BOMANI project, and the UK-based Born in Bradford study collectively represent a major step forward towards achieving this goal. References Bittles AH. (2001). Consanguinity and its relevance to clinical genetics. Clinical Genetics 60, 89-98. Bittles AH. (2008). A Community Genetics perspective on consanguineous marriage. Community Genetics 11, 324-330. Bittles AH. (2012). Consanguinity in Context. Cambridge University Press. Bittles AH, Black ML. (2010) Consanguinity, human evolution and complex diseases. Proceedings of the National Academy of Sciences, USA 107, 1779-1786. Bittles AH, Grant JC, Sullivan SG, Hussain R. (2002). Does inbreeding lead to decreased human fertility? Annals of Human Biology 29, 111-131. Bittles AH, Mason WM, Greene J, Appaji Rao N. (1991). Reproductive behavior and health in consanguineous marriages. Science 252, 789-794. Cavalli-Sforza LL, Moroni A, Zei G. (2004). Consanguinity, Inbreeding, and Genetic Drift in Italy. Princeton: Princeton University Press. Hamamy H, Antonarakis SE, Cavalli-Sforza LL. et al. (2011). Consanguineous marriages: pearls and perils: Geneva International Consanguinity Workshop Report. Genetics in Medicine 13, 841-847. Ottenheimer M. (1996). Forbidden Relatives – the American Myth of Cousin Marriage. Chicago: University of Illinois Press. Schull WJ, Neel JV. (1965). The Effects of Inbreeding on Japanese Children. New York: Harper and Row.

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abstract 12Bologna_Bittles_Consanguinity_facts_vs_fiction_GF_CC_5217.pdf

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A. Bittles. Consanguineous marriage: facts versus fiction. EUROGENE portal. July 2012. online: http://eurogene.open.ac.uk/content/consanguineous-marriage-facts-versus-fiction

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