Hemoglobin Genotype and Fertility in a Malarial Environment: Limon, Costa Rica

Abstract

This dissertation tests the hypothesis of differential fertility of Hb AS females in a malarial environment. The biocultural factors that affect achieved reproduction in this sample are examined. The aim of the research is to document the action of natural selection through differential fertility, and the interaction of culture and biology to produce achieved fertility. Epidemiological, clinical and in vitro studies indicate that Hb AS individuals experience low malarial morbidity and mortality. Thus, natural selection increases the frequency of hemoglobin S through differential mortality when the selective agent is malaria. However, it has also been proposed (Livingston, 1957) that the high frequency of hemoglobin S in malarial environments may be the result of differential mortality and reproduction. Pirscheinfs work (1961, 1984) suggests that Hb AS women have higher fertility in a malarial environment because Hb AA mothers suffer more frequent abortions as a result of placental malaria. Fertility data and blood sample were collected from 159 black women from the coastal town of Limon, Costa Rica. This investigation indicates that there are no significant differences between Hb AS and Hb AA mothers in terms of live births (t=-0.69, ns, df=98), completed family size (t=0.79, ns, df=98) or abortions (t=0.61, ns, df=93). The analysis of the reproductive data identifies the main biocultural factors that affect achieved fertility in the Limon population. In this sample, age at menarche is significantly correlated with the number of abortions (r=-0.28, p< 0.005) indicating that early menarche increases the risk of fetal wastage. Women with completed family size above average are different from their peers only in the number of pregnancies and livebirths. A path analyses of the fertility data allows the examination of the causal structure of achieved fertility in the sample. The path analyses indicates that the number of multiple births and abortions influence the completed family size mainly through their correlation with the number of pregnancies. In conclusion, this dissertation does not support the hypothesis of differential reproduction of Hb AS and Hb AA females in a malarial environment. This study confirms prior indications that the Hb S polymorphism has been maintained solely through differential mortality. In contrast with previous research, this dissertation incorporates biological and cultural components of completed fertility. Achieved fertility in Limon is influenced by the age at menarche, the number of pregnancies, and the number of abortions. The age at marriage does not influence the reproductive outcome.