Julio knows that sickle cell disease is a serious genetic disorder. He learns that in the United States, approximately 1 in 12 people of African ancestry carries the sickle cell allele. He asks, "Why is the sickle cell allele present in such a high frequency?" Which statement would be MOST USEFUL to include in an answer to Julio's question?

A. The sickle cell allele causes a significant change to a blood protein.
B. The sickle cell allele formed from a point mutation.
C. Two copies of the sickle cell allele are necessary to express the disorder.
D. Having one copy of the sickle cell allele provides resistance to malaria.