Heterozygote advantage

Heterozygote advantage is the situation where, at a gene locus, the heterozygote has greater fitness than either homozygote.p205 It is a type of balancing selection.
Sickle cell anaemia as an example.
Hb+ = gene for normal haemoglobin molecules; Hbs = mutant gene. Three genotypes are possible in a population: Hb+Hb+ producing a normal phenotype; Hb+Hbs the heterozygote; and HbsHbs homozygous for sickle cell anaemia.
Fitness.
The fitness of these three genotypes varies according to the environment. In the temperate zone, the normal homozygotes and the heterozygotes will leave most offspring. This is because the homozygote for sickle cell has less chance of surviving to leave children, and if they do survive, they will have many health problems.
In a tropical climate, such as equatorial Africa, malaria is a big killer. There, the heterozygote has the advantage in fitness. They do not get malaria so badly, and they do not suffer the earlier deaths of the homozygote sickle cell sufferers.
Not surprisingly, the Hbs gene is present in about a quarter of Africans from the countries near the equator. In Europeans it is a rare gene. 
Polymorphism.
Heterozygote advantage is probably the main cause of polymorphism in biological species. Polymorphism, alternate genetic types in a population (for example different blood groups), is extremely common. The fitness of the heterozygotes keeps a wide range of alternate gene alleles in a population. This, in turn, gives the population a wide range of genetic 'resources' and this gives a better chance of survival.