Problems of Marriages between blood relations - Consanguineous Marraige

Consanguineous marriages, or marriages between blood relatives, are fairly common within certain cul¬tures. Unfortunately, children from these types of marriages may be at a very elevated risk for genetic illness. For a child to inherit a recessive disease, both parents must be carriers.

For a rare recessive disorder, where the chance of any individual being a carrier is one in 1000, the odds of both parents being carriers would be 1 in 1,000,000 (1/1000 x 1/1000). The chance that any given fetus will inherit defective genes from both parents is 1 in 4,000,000. (1/4 x 1/1000).


Let's instead consider two parents that are first cousins. The most likely way that they would both be carriers would be if one of their grandparents was a carrier, and had passed the disease gene to both of their parents. Since there are two great-grand¬parents at the top of the consanguinity loop, the chance that one would be a carrier would be 2/1000 (2 x 1/1000), There is a 1/2 chance that any given child will inherit the defective gene, so the odds that both the grandparents would be carriers would be 1/4 (1/2 x 1/2), assuming that one great-grandparent was a carrier (remember there is a 2/1000 chance of that) The odds that both parents would be carriers, again assuming that their parents were carriers, is again 1/4 (1/2 x 1/2).

Finally, the chance that any child of theirs would be affected is 1/4, assuming they are both carriers. When we multiply out the proba¬bilities of each of these people being carriers and a child being affected, we get 1/32,000 (2/1,000 x 1/4 x 1/4 x 1/4). While this is quite a low number, com¬pared to the risk for a child from a non-consanguineous marriage (1/4,000,000), the risk to the child from the consanguineous marriage is increased by 125-fold.