Anthropological Implications of Sickle Cell Gene Distribution in West Africa1

URING the past fifteen years, data on the frequency of the sickle cell D gene have accumulated to such an extent that its world distribution can now be outlined in considerable detail. Frequencies of more than 20 percent of ‘the sickle cell trait have been found in populations across a broad belt of tropical Africa from the Gambia to Mozambique. Similar high frequencies have been found in Greece, South Turkey, and India. At first it appeared that there were isolated “pockets” of high frequencies in India and Greece, but more recently the sickle cell gene has been found to be widely distributed in both countries (Choremis and Zannos 1956; Sukumaran, Sanghvi, and Vyas 1956). Moreover, between these countries where high frequencies are found, there are intermediate frequencies, in Sicily, Algeria, Tunisia, Yemen, Palestine, and Kuwait. Thus, the sickle cell gene is found in a large and rather continuous region of the Old World and in populations which have recently emigrated from this region, while it is almost completely absent from an even larger region of the Old World which stretches from Northern Europe to Australia. When the broad outlines of the distribution of the sickle cell gene first began to emerge, several investigators attempted to explain various aspects of this distribution by migration and mixture. Lehmann and Raper (1949) attempted to show that the differences in the frequency of the sickle cell gene among the Bantu tribes of Uganda were due to varying degrees of Hamitic admixture; Brain (1953) and Lehmann (1954) postulated migrations from Asia to account for the distribution of the sickle cell gene in Africa; and Singer (1953), using an age-area type of argument, postulated that the sickle cell gene arose by mutation near Mt. Ruwenzori and diffused from there. However, it was recognized early in the development of the sickle cell problem that regardless of the extent to which migration and mixture explained the distribution pattern of the sickle cell gene, its high frequencies in various widely scattered areas raised some additional and striking problems in human population genetics. Since persons who are homozygous for the sickle cell gene very rarely reproduce, there is a constant loss of sickle cell genes in each generation. In order for the gene to attain frequencies of .1 to -2, which are equivalent to about 20 to 40 percent of the sickle cell trait, there must be some mechanism which is compensating for this loss. In other words, there must be some factor which is tending to increase the number of sickle cell genes in the population. Nee1 (1951) first pointed out that there are two outstanding possibilities;