Marital mores as a mechanism for the maintenance of ethnic variations of lethal gene frequencies.

A computer simulation model was developed to study the effects of various marital mores on the incidence of lethal autosomal recessive genes in populations that are subdivided into small isolates. The problem was studied in isolates where initial generation size was 30, 40, and 50 individuals. In each of these, the mean fertility rate was varied from 2.3 to 2.7 surviving (to adulthood) children per couple whose marriage had been contracted in accordance with the prevailing convention: marriage between first cousins and siblings prohibited; marriage between siblings prohibited; marriage allowed between any individuals; marriage prohibited between siblings but encouraged between cousins; and marriage encouraged between siblings. In all cases, the mean gene frequency in generation 20 was lower than that predicted by the deterministic model with random mating in an unsubdivided population of infinite size, due to gene loss through random drift (to zero) in many of the isolates. The mores that encouraged consanguineous marriages had the lowest final lethal-gene frequencies. Random mating produced intermediate values, and the restrictive mores, the highest final frequencies. The deterministic model (assuming infinite population size and random mating) predictions of the final gene frequency were exceeded only if there was reproductive compensation. It is concluded that restrictive marital mores significantly reduce the selective pressures on lethal recessive genes in small isolates, but that this is counteracted by the increased rate of gene loss through random drift.