Final answer:
Overdominance, or heterozygous advantage, refers to the phenomenon where the heterozygous genotype has greater fitness than either homozygous form. This leads to an increase in the frequency of heterozygotes (2pq) in a population. Overdominance facilitates polymorphism and is different from codominance or simple dominance.
Step-by-step explanation:
Overdominance, also known as heterozygous advantage, is a genetic phenomenon where the heterozygous genotype (having two different alleles at a gene locus) has a higher relative fitness than either the homozygous dominant or homozygous recessive genotypes.
In a population experiencing overdominance, one would expect the value of 2pq (which represents the frequency of the heterozygous genotype in the Hardy-Weinberg equation) to increase. This is because natural selection would be favoring individuals with the heterozygous condition, leading to a higher proportion of these individuals in the population over time.
Overdominance is different from codominance, where both alleles for a gene are fully expressed in the phenotype of the heterozygotes, such as the MN blood group in humans. Unlike dominance where a dominant allele masks the effect of a recessive allele, overdominance results in a distinct advantage for the heterozygote that cannot be attributed to either allele alone.
This can lead to a stable form of polymorphism in the population, where multiple alleles are maintained. An example of overdominance is the resistance to malaria seen in individuals who are heterozygous for the sickle cell trait.