Balanced Polymorphism

Mutations that cause recessive diseases arise and are maintained in populations in two ways:

1. consanguinity

2. balanced polymorphism

Balanced polymorphism also maintains and produces these recessive disease-causing mutationsBalance polymorphism is the ability of natural selection to maintain diversity in a population

Balancing selection = balanced polymorphism or balancing selection becomes balanced polymorphism because balancecd polymorphism is the state at which balancing selection occursBalancing selection is defined as: Natural selection that maintains stable frequencies of two or more phenotypic forms in a population and this state is balance polymorphism

Balancing selection can maintain the frequencies of two or more phenotypes in a population, resulting in balanced polymorphism. Balancing selection includes heterozygote advantage and frequency-dependent selection:

Heterozygote advantage: 

1. Heterozygous individuals have a greater reproductive success

2. Heterozygote advantage tends to preserve variation in gene pools

3. When individuals are heterozygous at a certain gene locus they have survival and reproductive advantages. Their heterozygote advantage tends to maintain two or more alleles at this locus. For examples, we see this heterozygote advantage in the gene for sickle cell disease. The heterozygote (carrier) for the disease enjoys immunity to malaria. The frequency of the sickle-cell allele in Africa is highest in areas where the malaria parasite is most common.

4. Frequency-Dependent Selection: 

5. The fitness of any one morph (fitness meaning reproductive success) declines if it becomes too common in the population  Frequency-dependent selection can include predation.  For example, frequency-dependent predation; as predators learn to focus on the more common prey items, polymorphism in prey populations are preserved

Balanced Polymorphism in AIDS:

Europe’s bubonic plague epidemic in the 1300s is considered to be a type of balanced polymorphism; people who are carriers to HIV but don’t inherit the full AIDS owe their lives to survivors of Europe’s bubonic plague.

References:

1) Biology. Seventh Edition, Neil A. Campbell and Jane B. Reece. Pearson Eduction Inc., publishing as Benjamin Cummings, 1301 Sansome St., San Franscisco, CA

2) Biology. Peter H. Raven, George B. Johnson, Susan Singer, Jonathan Losos. McGraw-Hill Science/Engineering/Math; 7 edition (January 8, 2004)

3) DNA Science: A First Course, Second Edition. David A. Micklos. Cold Springs Harbor Laboratory Press; 2 edition (January 8, 2003)