Cousin marriage: Marriage involving cousins.

Everyone carries rare recessive alleles, rare genes that are generally innocuous in the heterozygous state but that in the company of another gene of the same type are capable of causing an autosomal recessive disease. We are all reservoirs for genetic disease. First cousins, as noted, share a set of grandparents. So for any particular allele (gene) in the father, the chance that the mother inherited the same allele from the same source is 1/8. Further, for any gene the father passes to his child, the chance is 1/8 that the mother has the same gene and ½ that she transmits that gene to the child so 1/8 X ½ = 1/16. Thus, a first-cousin marriage has a coefficient of inbreeding of 1/16. The added risks for first cousins depend not only upon this coefficient of inbreeding but also upon their genetic family histories and, in some cases, upon test results (for example, for beta thalassemia for first cousins of Italian descent). However, there are always added risks from the mating of closely related persons and those risks are not negligible.