2. Balanced Genetic Polymorphism:

• Definition: Balanced polymorphism occurs when two or more alleles are maintained in a population at relatively stable frequencies over many generations. This stability is typically due to some form of balancing selection, where no single allele has a consistent, overwhelming advantage.
• Characteristics: Allele frequencies remain relatively constant because the selective forces favor the maintenance of multiple alleles. The polymorphism is long-lasting.
• Underlying Selection: Often involves:
  • Heterozygote Advantage (Overdominance): The heterozygote genotype has a higher fitness than either homozygote.
  • Frequency-Dependent Selection: The fitness of a genotype depends on its frequency in the population.
  • Varying Selection Pressures: Different alleles are favored in different environments or at different times.
• Example from Human Populations: Sickle Cell Trait and Malaria Resistance. This is a classic example of heterozygote advantage. The gene for beta-hemoglobin has two main alleles: HbA (normal) and HbS (sickle cell). Individuals homozygous for HbA (HbA/HbA) are susceptible to malaria. Individuals homozygous for HbS (HbS/HbS) suffer from severe sickle cell anemia, which is often fatal without modern medical intervention. However, individuals heterozygous for the trait (HbA/HbS) are resistant to malaria and generally do not suffer from severe anemia. In regions where malaria is endemic, the HbA/HbS heterozygotes have the highest fitness. This selective pressure maintains both the HbA and HbS alleles in the population, even though the HbS/HbS homozygote is severely disadvantaged. The frequencies of HbA and HbS reach an equilibrium where the advantage of the heterozygote balances the disadvantage of the HbS homozygote, thus creating a balanced polymorphism.