• 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
• Illustration
• 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)

• single nucleotide polymorphisms (SNPs)
• small-scale insertions/deletions
• polymorphic repetitive elements
• microsatellite variation