Heredity, Natural Selection, and Evolution

Species and Population

What is balanced polymorphism?

When a trait exists in several forms within a population, it is said to be polymorphic. Polymorphisms that maintain a stable distribution within the population over generations are known as balanced polymorphisms. Balanced polymorphism can be maintained if heterozygotes (mixtures of two types) have a fitness advantage. When this occurs, both types of alleles are maintained in the population. Strangely enough, a classic example of this is sickle cell anemia and malaria: Individuals who are heterozygous (Hh) are resistant to malaria, dominant homozygotes (HH) are susceptible to malaria, and recessive homozygotes (hh) have sickle cell anemia. Because those who have both types of alleles and who live in malaria-prone regions are the most likely to survive long enough to produce children, both types are maintained in the population at a relatively stable rate.


This is a web preview of the "The Handy Biology Answer Book" app. Many features only work on your mobile device. If you like what you see, we hope you will consider buying. Get the App