Effects of consanguineous marriages on morbidity and precocious mortality: genetic counseling.

The excess risks of morbidity and precocious mortality for the offspring of incestuous matings and of matings of uncles-nieces and aunts-nephews, first cousins, first cousins once removed, and second cousins have been estimated as 32%, 18%, 9%, 5%, and 2.5%, respectively. These estimates are based on the theory of Morton et al [1956], assuming a damage (genetic and nongenetic) of 20% for the offspring of nonconsanguineous couples and two "deleterious" equivalents per gamete. Other possibilities (a damage of 40%; 2.5 deleterious equivalents per gamete), a partition of the total risk into prenatal and postnatal events, and some aspects of the inbreeding theory are also presented. Comments intended for persons interested in counseling are provided.