Carriers of Sickle Cell Are Resistant to Malaria, and Carriers

Carriers of sickle cell are resistant to malaria, and carriers of cystic fibrosis are resistant to cholera. Both are the result of single gene mutations. Why is more than a single trait affected?

A) Genes for one trait can interact with and influence other traits as well.
B) Sickle cell and cystic fibrosis are caused by the same alleles.
C) Most genetic diseases are bad but have good characteristics, too.
D) Sickle cell disease is known to be related to intestinal diseases.
E) Malaria is also an infectious intestinal disease.