In this, Hbs genes may be lost from the population each time a homozygote (who has sickle cell disease) fails to contribute to the next generation because of death before the reproductive age. The heterozygotes (HBs/HBa) are, however, thought to be differentially maintained in the population because individuals with this phenotype are less likely to succumb to falciparum malaria and consequently survive to contribute genes to the next generation. The theory implies that there are different selective values to the several forms of polymorphisms.