The decline in survivorship is due to several factors that directly affect the childhood mortality. The dissimilarity between the genotypes can be described as variation in resistance to the infection by Plasmodium falciparum (malaria parasite), which is a main source of early childhood mortality in the regions where malaria is common. A study based on nearly 300 children assists to identify the level of parasites in the blood of the heterozygous AS children against wild-type AA children. The outcome of the study shows the occurrence of malarial parasites was certainly lower in AS children (27.9%) than in AA children (45.7%) and that parasite concentration was also lesser in AS children. The findings reveal that AS children had lower frequency and seriousness of malarial infection.
From the figure 20-7, AA homozygotes have the second highest mortality and have no sickling, but are subjected to great childhood mortality from infection by malaria parasites. AS exhibits lowest mortality rate and does not show harmful sickling, as the heterozygotes are partly resistant to the infection by the malaria parasite. One possible explanation for the decrease in the variation between AS and AA is the highest malaria mortality rate among the young children who are greatly susceptible to the infection due to the undeveloped natural immunity. Another explanation is the AA genotypes that stay alive could have acquired immunity to the malarial infection and are less prone to be infected as adults.