When two heterozygotes mate, of their offspring are predicted to be SS and have sickle cell anemia, AA normal, and AS, carrying the sickle allele. Malaria and sickle cell disease cover the same geographic areas in the world, and is caused by four species of Plasmodium, a protozoan blood parasite of humans that uses the female mosquito Anopheles .spp as its vector. Malaria also can be found in isolated local areas, such as lowland areas where breeding habitats are more abundant for Anopheles to thrive, but usually not in mountainous areas where Anopheles breeding habitats are less common. If malaria is found in other areas, it s usually the result of the transportation of Anopheles. The frequency of the sickle cell gene is quite high in malaria stricken areas.
Gene mutations occur all the time, but at low rates.
Any mutation that is not advantageous to a species is found at very low frequencies in a population. Sickle cell trait, by itself, is not very advantageous to a general population because of its debilitation and mortality rate. Since death for homozygous recessive individuals and homozygous normal individuals can occur after reaching reproductive age, any progeny will inherit at least one allele for sickle cell and one normal allele if homozygous recessive and dominant parents mate. Since both homozygous types have a shorter life span due to the effects of either sickle cell or malarial infection, over time there are fewer homozygotic individuals inhabiting a given area. Any progeny inheriting one normal allele and one sickle cell allele become heterozygous for sickle cell trait, and are considered carriers of sickle cell.