Cousin Marriages - Consanguineous Marriages and Health Risks - Marrying a Close Relative

Consanguineous marriages have long been practiced in different parts of the world. Though the practice emerged out of certain practical and financial advantages, modern science found several instances of such marriages bearing health risks for the next generation. Here is a brief guide on consanguineous marriages and associated health issues.

Plainly put, consanguineous marriages are those where the partners are related by blood or in other words descended from the same ancestor. Consanguinity is an important legal concept since laws in certain societies consider consanguinity while considering if two people can be married or how a deceased person’s property may be inherited in case of absence of will.

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In United States, certain states prohibit consanguineous marriages by law. There are various degrees of consanguinity and while some states may simply disallow marriages to brothers, sisters, fathers and mothers, other states extend the prohibition to first cousins as well.  The Canon law of the Roman Catholic Church in fact annulled marriages between first cousins and banned marriages within the fourth degree of a consanguineous relationship since 1215. And even though dispensations could be granted to get around legal barriers, they became harder to get the closer the couple were related.

According to genetics, consanguinity is defined according to the amount of shared identical DNA or the genetic material between two individuals. The percentage of consanguinity between the two decreases four times for every generation that their most recent common ancestor is removed. For instance first cousins will have four times the consanguinity as second cousins.  For all practical purposes, consanguineous unions are defined as those which are contracted between biologically related second cousins or nearer. In scientific terms, this refers to partners who share the inbreeding coefficient of F 0.0156 or more.

The primary reason why consanguineous relationships receive such attention in law and genetics is because they have long been held to be unsafe for the health of future generations. Several studies have revealed the offspring of consanguineous marriages to be more prone to certain genetic disorders like autosomal recessive disorders. Since blood relatives share a proportion of their genes, the related parents or the consanguineous couple may be the carriers of an autosomal recessive gene. While the carriers may not be aware of or suffer from any health problems, their offspring may reveal symptoms of an autosomal recessive disorder. In fact the closer the parents are related the greater is the risk of their offspring suffering from the genetic disorder, according to H. M. Kingston, author of “ABC of Clinical Genetics”.

A 1994 study titled “The costs of human inbreeding and their implications for variations at the DNA level” and published in Nature Genetics(8) found that inbreeding at the first cousin level led to a mean excess mortality of 4.4%. Also empirical studies on the progeny of first cousins indicate morbidity levels to be one to four percent higher than in case of offspring from unrelated couples, according to a 1988 study by A. H. Bittles and U. Makov titled, “Inbreeding in human populations: assessment of the costs”. Bittles also points out that the less common a genetic disorder in the general population, the higher the probability of its appearance in societies where consanguineous relationships are common. In fact many of previously unknown genetic disorders have been first reported from highly endogamous societies and in a high proportion of cases the genetic mutation causing the disorder is also unique to the community.

Yet another health risk for children of consanguineous unions is a greater susceptibility to infectious diseases. A study jointly conducted by E. J. Lyons, A. J. Frodsham, L. Zhang, A. V. Hill and A Amos in 2009 and titled “Consanguinity and susceptibility to infectious diseases in humans” found that offspring of closely related couples who have low genetic heterozygosity are more prone to suffer from infectious diseases like tuberculosis and hepatitis. Using microsatellite genome screen data for tuberculosis, hepatitis and leprosy, the authors found that inbred individuals were more common in infected cases of tuberculosis and hepatitis but only in populations which have a high degree of consanguineous marriages. Countries like Italy where consanguineous unions are rare were relatively free from inbred cases of hepatitis infection while no effect was found for leprosy which is thought to be oligogenic. The results led to the researchers to conclude that consanguinity is an important risk factor for human susceptibility to infectious diseases.

However the tradition of consanguineous marriages in some cultures and its prevalence in certain parts of the world till today indicate that the practice must have possessed certain advantages. Globally the most common form of consanguineous union is that of between first cousins in which the partners share one-eighth of their genes inherited from a common ancestor. Countries like Pakistan and Turkey have large percentages of consanguineous marriages which owe their prevalence to cultural and religious considerations. On the other hand minority social groups in Western countries like USA and Australia may continue to practice consanguineous marriages out of desire to find marital partner from within the same community which may itself have few kin-groups.

Many of the reported ill-effects of consanguineous marriages need to be taken in the context of demographic and socio-economic factors too. For instance Bittles points out that in a country like Pakistan where there is a significant percentage of consanguineous marriages, reasons for infant mortality can range from maternal illiteracy, low age of women during childbirth as well as a birth interval of less than eighteen months. But even after taking these factors into account Bittles and Grant in their 1997 study “The comparative role of consanguinity in infant and child mortality in Pakistan” published in the Annals of Human Genetics 61 found that first cousin progeny to have statistically significant risks of neonatal, post-neonatal and infant mortality of 1.36%, 1.28% and 1.32% respectively.

Health risks of consanguineous marriages is a wide and complex issue with legal, medical as well as social implications. The rise of identity politics in various parts of the world has also added another dimension to the issue of what can and cannot be acceptable in society. The need of the hour is a larger number of studies which would be able to better estimate the extent of consanguineous unions across world populations and study its impact in a multi-disciplinary context.