Question

1. Deleterious alleles can be maintained in a population when carriers are protected against certain unrelated, but common disease. This situation is described as:

   A) Genetic drift B) Heterozygote advantage C) Hardy-Weinberg equilibrium D) Polymorphism E) Genetic admixture

Answer 1

Ans.) Option B : Heterozygote Advantage

Explanation : There are certain examples of deleterious genes which are usually lethal in homozygous condition. But when these genes are present in heterozygous condition, this heterozygous alleles favours relatively higher fitness to individual than homozygous dominant and homozygous recessive.

For example, Sickle cell anemia is lethal when mutated alleles are present on both the locus of homologous pairs. Mutated allele which leads to sickle anemia condition causes abormal shape to the RBCs, it is no longer circular disc shaped but gets transformed into sickle shape. Due to this cahnge in shape the O2 carrying capacity decreases and anemic condition occurs. Sickle cell mutated allele have co-dominant effect to normal allele. So in heterozygous condition half of the RBC would be of normal shape and other half of sickle shape.

Heterozygous population for sickle cell anemia have heterozygous advantage against malaria. Malaria is also wide spread and can be deadly if not treated. malaria is caused by Plasmodium parasite.  Plasmodium after entering the body, attaches to RBCs and enters inside it and ultimately causing its breakdown. Heterozygous population due to partial distorted shape of RBCs the pathogen is unable to identify it and donot enter the RBCs. So in heterozygotes the partial distorted RBC still is able to carry O2 and gets natural resistance against malrial pathogen too.

In homozygous recessive, sickle cell anemic condition is quite considerable and hence these individual die after few year of birth because of anemia.

In homozygous dominant, the individuals are susceptible for malaria and death rate in such individuals is high due to malaria.