DIFFERENTIAL FERTILITY AS A MECHANISM MAINTAINING BALANCED POLYMORPHISMS IN SARDINIA

Women's fertility, gathered from the 1961 Italian population census, a nd estimates of heterozygote frequencies for thalassemia and G6PD defi ciency (Siniscalco et al. 1961, 1966) in 52 Sardinian villages were ex amined to study at the population level the mechanisms that have maint ained the stability of these polymorphisms over long periods. Sardinia n villages were classified according to low or high frequency of heter ozygotes, and the reproductive behavior of the women living in these a reas was analyzed. A high mean number of children per woman and a low percentage of women without children with a high heterozygote frequenc y was demonstrated. The observed differential fertility and sterility were interpreted as being the result of different numeric ratios withi n each area between normal homozygous and heterozygous women, who were less and more resistant, respectively, to malarial infection, accordi ng 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 fe rtility and sterility by heterozygote frequency level and malarial mor bidity level. A combined and direct effect of inherited and acquired i mmunities on fertility and sterility rates was shown. The level of end emicity in an area may contribute to decreasing or increasing fitness, which is already influenced by the stable balanced polymorphisms.