reproducing and thereby passing the gene on to the next generation), the result of any cross between the two populations will be fitter than the parent. This article deals with the specific case of fitness overdominance, where the fitness advantage of the cross is caused by being heterozygous at one specific locus alone. Experimental confirmation Cases of both homozygote and heterozygote advantage have been demonstrated in several organisms, including humans. The first experimental confirmation of heterozygote advantage was with Drosophila melanogaster, a fruit fly that has been a model organism for genetic research. In a classic study on the ebony mutation, Kalmus demonstrated how polymorphism can persist in a population through heterozygote advantage. If weakness were the only effect of the mutant allele, so it conveyed only disadvantages, natural selection would weed out this version of the gene until it became extinct from the population. However, the same mutation also conveyed advantages, providing improved viability for heterozygous individuals. The heterozygote expressed none of the disadvantages of homozygotes, yet gained improved viability. The homozygote wild type was perfectly healthy, but did not possess the improved viability of the heterozygote, and was thus at a disadvantage compared to the heterozygote in survival and reproduction. This mutation, which at first glance appeared to be harmful, conferred enough of an advantage to heterozygotes to make it beneficial, so that it remained at dynamic equilibrium in the gene pool. Kalmus introduced flies with the ebony mutation to a wild-type population. The ebony allele persisted through many generations of flies in the study, at genotype frequencies that varied from 8% to 30%. In experimental populations, the ebony allele was more prevalent and therefore advantageous when flies were raised at low, dry temperatures, but less so in warm, moist environments. Heterozygote disadvantage occurs when "a heterozygote has a lower overall fitness than either homozygote." Heterozygote disadvantage occurs in mammals, birds, and insects. In human genetics Sickle-cell anemia Sickle-cell anemia (SCA) is a genetic disorder caused by the presence of two incompletely recessive alleles. When a sufferer's red blood cells are exposed to low-oxygen conditions, the cells lose their healthy round shape and become sickle-shaped. This deformation of the cells can cause them to become lodged in capillaries, depriving other parts of the body of sufficient oxygen. When untreated, a person with SCA may suffer from painful periodic bouts, often causing damage to internal organs, strokes, or anemia.