Over dominance

• The phenotypic value for the heterozygote is greater or less than the value for either homozygote.
• There aren’t many examples of this type of gene action. Sickle-cell anemia in humans would be one example.
• If untreated, individuals with sickle-cell anemia usually die at a fairly young age.
• Heterozygotes do not experience this condition but seem to have greater resistance to malaria than homozygous normal individuals. Thus, in regions of the world where malaria is present, heterozygotes have a higher fitness value than either homozygote. An example of overdominance for a measurable trait would be:

AA = 2 units, Aa = 3 units and aa = 1 unit.