Balanced Polymorphism: two or more alleles within a population that are maintained at almost a constant frequency becuase of the selective advantages of both.
Examples: Cystic Fibrosis
Carriers of Cystic Fibrosis (CF) had an advatange in our human history when cholera epidemics wiped out large percentages of populations. Cholera is caused by a bacteria that opens chloride channels and releases water causing life threatening diarrhea. CF closes choride channels, preventing the bacteria to grow. Carrier of CF balance mild effects of CF but were protected against cholera.
Video: Sickle Cell Anemia
Another example of balanced polymophism, where sickle cell anemia helps prevent malaria.