It is said, that the harmful alleles get eliminated from population over a period of time, yet sickle-cell anaemia is persisting in human population. Why?

It is said, that the harmful alleles get eliminated from population over a period of time, yet sickel cell anaemia is persisting in human population. Why?

How would you account for the occurrence of sickle-cell anaemia in human population?

It is expected that selection pressure should have eliminated the gene for sickle-cell anaemia. How would you account for its continuance in human population ?

Sickle cell anemia has not been eliminated from the African population because

____ protects us from getting the diseases over a long period of time .

"The population of a metro city experiences fluctuations in its population desity over a period of time." If 'N' is the population density at the time 't' , write the population density at the time 't+1'.