If a greater proportion of monozygotic twin pairs are found to be concordant for a certain disease than are dizygotic twin pairs, the observation suggests that the disease is most likely caused by:
a. Exclusively environmental factors
b. Exclusively hereditary factors
c. Hereditary factors almost exclusively, with some nonhereditary factors possibly playing a role
d. Environmental and genetic factors almost equally
e. Gender differences in monozygotic twins
Answer: c. Hereditary factors almost exclusively, with some nonhereditary factors possibly playing a role