Overdominance

Overdominance is a genetic phenomenon that occurs when a heterozygous individual (having two different alleles at a particular gene locus) exhibits a phenotype that is more pronounced or advantageous than that of either homozygous individual (having two identical alleles at a particular gene locus). This phenomenon is also known as heterozygote advantage or heterosis.

In the context of genetics, overdominance is often observed in cases where the heterozygous genotype confers a selective advantage in terms of fitness or survival compared to the two homozygous genotypes. This can result in the maintenance of genetic diversity within a population, as both alleles are maintained at relatively high frequencies due to the selective advantage of the heterozygous genotype.

One classic example of overdominance is sickle cell anemia, a genetic disorder caused by a mutation in the hemoglobin gene. Individuals who are homozygous for the sickle cell allele have sickle cell disease, which can lead to serious health complications. However, individuals who are heterozygous for the sickle cell allele (carriers) have a selective advantage in regions where malaria is prevalent, as they are less susceptible to the disease. This results in the maintenance of both the normal and sickle cell alleles in the population.

Another example of overdominance is the major histocompatibility complex (MHC) genes in vertebrates. The MHC genes encode proteins that play a crucial role in the immune response by presenting antigens to T cells. Heterozygous individuals with a diverse set of MHC alleles are better able to recognize and respond to a wide range of pathogens, providing them with a selective advantage in terms of immune defense.

In agricultural genetics, overdominance is exploited in breeding programs to produce hybrid crops with superior traits such as increased yield, disease resistance, or stress tolerance. By crossing two genetically distinct parental lines, breeders can take advantage of the heterosis effect to create hybrids that outperform their parents in terms of productivity and resilience.

Overall, overdominance is a fascinating genetic phenomenon that highlights the importance of genetic diversity in populations and the complex interactions between alleles in determining phenotypic outcomes.