The phenomenon by which one gene controls many traits is called:

The correct option is A

pleiotropy

Often, an individual allele will have more than one effect on the phenotype. Such an allele is said to be pleiotropic. In pleiotropy, one gene affects many traits. Pleiotropic effects are difficult to predict, because the genes that affect a trait often perform other functions we may know nothing about. Pleiotropic effects are characteristic of many inherited disorders, such as cystic fibrosis and sickle cell anemia.