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Why are consanguineous marriages not advisable?

In consanguineous marriages (e.g. first cousin and uncle niece marriages), there is an increased risk in the offspring of traits controlled by recessive genes and those determined by polygenes. Examples are albinism, alkaptonuria, and phenylketonuria.

An increased risk of premature death, mental retardation and other birth defects are also noted in such children.

The basic of this can be explained by the following example. In albinism, which has autosomal recessive inheritance, the incidence is 1 in 20,000 live births. Let us say, they are of `aa' genotyes. Then the remaining 19,999 are either `AA' (Normal) or `Aa' (Carriers). We can calculate the frequency of carriers by using `Hardy-Weinberg law', which will give the frequency as 280 carriers for each albino patient.

In the normal out breeding pattern of society, this ratio holds good. However the probability of wedding of two carriers (for the development of autosomal recessive conditions, both the parents must be carriers) is increased in consanguineous marriages, with resultant increase in the frequency of genetic diseases in their children.

This has been proved by many number of epidemiological studies. The following are the two examples.

• Phenylketonuria has an incidence of 1 in 25,000 in the offspring of unrelated marriages, but it is 13 per 25,000 in children of first cousin marriages. (13 fold increase).
• In a Japanese city, a death rate of 116 per 1000 was found during the first 8 years of life amongst the offspring of first cousin marriages against 55 per 1000 amongst the control normal population.

Therefore, a reduction of consanguineous marriages would be advantageous to the health of the community.