Sickle-Cell Disease Is a Recessive Genetic Disease

Sickle-cell disease is a recessive genetic disease. This means

A) two copies of the recessive allele must be present in order for the disease to be expressed.
B) only one copy of the recessive allele must be present in order for the disease to be expressed.
C) at least one copy of each recessive allele must be present for each gene involved in sickle-cell disease in order for the disease to be expressed.
D) the environment can alter the expression of recessive genes, meaning any combination of alleles can cause the disease if the right environmental conditions are encountered.