Sickle cell anemia is a disease that affects the shape of red blood cells in individuals who have two copies of the sickle cell allele. Individuals with one copy, called carriers, do not exhibit sickle cell anemia but are resistant to the disease malaria, which is spread by mosquitoes. The top map shows where malaria is common, while the bottom map shows regions where the sickle cell allele is most prevalent. Which of the following statements might account for the prevalence of the sickle cell allele in the areas shown?
a
Sickle cell is a polygenic trait, so it must be present on many chromosomes.
b
The allele is advantageous in cold areas uninhabited by mosquitoes.
c
Sickle cell is caused by chromosomal nondisjunction.
d
The allele is advantageous in tropical areas where the risk of malaria is the greatest.