THE SICKLE‐CELL and HAEMOGLOBIN C GENES IN SOME AFRICAN POPULATIONS
Authors: ALLISON A. C. 
Published: September 2007

1. The presence or absence of sickling of red blood cells and electrophoretic mobility of haemoglobin was investigated in 729 infants between 2 months and 1 year of age and 3602 parents and other adults from various East and West African territories. The distribution of the sickle‐cell and haemoglobin C genes in the Gambia, Sierra Leone, Nigeria and the Gold Coast is described.

2. In the district of Musoma, Tanganyika, and in various populations from the Gambia the incidence of sickle‐cell homozygotes in infant populations was found to be close to that expected from the Hardy‐Weinberg law. In the adults, however, there was a highly significant deficiency of homozygotes. From available data it is estimated that the viability of the sickle‐cell homo‐zygote is of the order of one‐fifth that of other genotypes in both East and West Africa.

3. The number of offspring of 176 matings between different genotypes from the Musoma district has been recorded. The mean total number of offspring is highest in sickle‐cell trait x sickle‐cell trait matings and least in normal x normal matings. The difference is, however, small and statistically insignificant. The mean number of dead offspring is highest in the trait x trait matings and least in the trait x normal matings. The differences, though greater than those observed between total offspring of the different matings, are not statistically significant at the 5 % level. About one‐third of all the children born in the Musoma district die within the first 4 years of life. It is concluded that the fitness of all genotypes between conception and birth is equal, and that the differences in fitness later appearing between the several genotypes are due mainly to differential survival between birth and reproductive age. Differences in adult fertility are relatively unimportant.

4. The sickle‐cell heterozygote frequencies in all but one of the populations tested were found to be higher in adults than in infants. In the Musoma district the higher frequency in adults is statistically significant at the 5 % level and of the right order of magnitude to keep the frequency of the sickle‐cell gene constant in spite of the elimination of four‐fifths of the homozygotes. In the Gambia the higher heterozygote frequency in adults is not significantly different from that in infants. It is concluded that the main factor maintaining the stable polymorphism of the sickle‐cell gene in East Africa is heterosis. Calculations of the mortality from malaria and other causes affecting the three genotypes in the Musoma district are given.

5. The high frequency of the haemoglobin C gene in the Gold Coast (about 6 %) is confirmed. Lower frequencies are recorded in Nigeria and Sierra Leone, and still lower frequencies in the Gambia. The gene appears to be very rare in, or absent from, the Southern Sudan, East Africa and the Belgian Congo. It is concluded that there is a single focus of the haemoglobin G gene in Africa, with a maximum frequency in or near the northern Gold Coast and a cline of descending frequencies on all sides. It is shown that there could be a stable polymorphism of the three alleles (HbA, Hbs and Hbc) in the Gold Coast under conditions of intensive selection where one heterozygote (HbsIHbc) is at a disadvantage and the other two heterozygotes are at an advantage.