Differential fertility as a mechanism maintaining balanced polymorphisms in Sardinia.

Women's fertility, gathered from the 1961 Italian population census, and estimates of heterozygote frequencies for thalassemia and G6PD deficiency (Siniscalco et al. 1961, 1966) in 52 Sardinian villages were examined to study at the population level the mechanisms that have maintained the stability of these polymorphisms over long periods. Sardinian villages were classified according to low or high frequency of heterozygotes, and the reproductive behavior of the women living in these areas was analyzed. A high mean number of children per woman and a low percentage of women without children with a high heterozygote frequency was demonstrated. The observed differential fertility and sterility were interpreted as being the result of different numeric ratios within each area between normal homozygous and heterozygous women, who were less and more resistant, respectively, to malarial infection, according to Haldane's theory. The effect of differing degrees of malaria on fertility rates has been demonstrated previously (Zei et al. 1990). To account for the effect of the genetic and epidemiological composition of an area on reproductive behavior, we classified data on women's fertility and sterility by heterozygote frequency level and malarial morbidity level. A combined and direct effect of inherited and acquired immunities on fertility and sterility rates was shown. The level of endemicity in an area may contribute to decreasing or increasing fitness, which is already influenced by the stable balanced polymorphisms.