Search

A computer simulation model was developed to study the effects of various feudal social customs on the incidence of lethal autosomal recessive genes. Populations of 500 individuals were created in which each person was characterized by sibship, genotype, and sex. The numbered sibships were sorted into numerical order. Each individual then sought a mate from 6 or 12 sibships on either side of his or her own sibship. The resulting couples were sorted in order of the husband's sibship number, after which the couples produced children in accordance with the prevailing birth rate. This ordering of sibships made the probability of a gene remaining in one locality higher than the probability of it drifting to distant localities. A lethal autosomal recessive gene was introduced into populations at a starting carrier rate of 10%. This fell after 20 generations to 2.54 +/- 0.17% (SEM) which was significantly less than the 5.00% carrier rate predicted by a deterministic model in which random mating occurred in an infinite population. The excessive loss of the lethal gene was caused by the high incidence of consanguineous marriages and by the occurrence of random inequalities in the distribution of the gene. The avoidance of sib marriages raised the final carrier frequency (3.28 +/- 0.19%) as did reproductive compensation (4.33 +/- 0.26%). The combination of the incest taboo and reproductive compensation raised the final carrier rate to 5.05 +/- 0.28%. It is concluded that the social factors which may have operated over a period of many centuries in the past have had a significant effect on the population genetics of lethal autosomal recessive genes.

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.