When a gene exists in two or more forms resulting in multiple genotypes over generations.- usually stable proportion.
Preconditions for balanced polymorphism
Frequency of allele should not be less than 1%
frequencies must be maintained naturally and not through recurrent mutations
Human red blood cells contain hemoglobin protein on their surface which carries oxygen to different parts of body. But some people have abnormal hemoglobin hb D, E,F,S etc
In some populations hbS frequency is maintained over generations because it creates sickle cell shaped red blood cells that provide resistance against malaria to heterozygotes but declines 10% oxygen carrying capacity which can be managed. But homozygous for hbA face malaria and HbS face shark decline in oxygen carrying capacity. Therefore polymorphism is maintained.
Found usually in Mediterranean and African populations.
It award about 10,000 years ago when forest was cleared for agriculture. Therefore environmental factors maintain polymorphism.
Genetic Variation (Polymorphisms)
Genetic variations are differences in DNA sequence among individuals that may underlie differences in health. Genetic variations occurring in more than 1% of a population would be considered useful polymorphisms for genetic linkage analysis. Various types of polymorphisms include: