Two parents who do not have sickle cell anemia have a child that has the disease. The parents are both:_______
A) Pleiotropic for the sickle cell allele
B) Homozygous for the sickle cell allele
C) Homozygous for the normal allele
D) Heterozygous for the sickle cell allele
E) Epistatic for the sickle cell allele