Marriage, cousin: A form of consanguinity. Everyone carries recessive alleles, 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 disease. We are all genetic reservoirs for genetic disease. Since first cousins share a set of grandparents, for any particular allele (gene) in the father, the chance that the mother inherited the same allele from the same source is 1/8. And 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 it to the child, so 1/8 X ½ = 1/16. A first-cousin marriage therefore 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 the risk of beta thalassemia in first cousins of Greek or Italian descent). There are always added risks from the mating of closely related persons.