A man who can roll his tongue and a woman who cannot roll her tongue have a son who can roll his tongue (R = can roll tongue; r = can't roll tongue). The son is curious about whether his father is homozygous or heterozygous for the tongue-rolling trait. Which of the following facts would allow him to know?

a. His paternal grandfather and his paternal grandmother can both roll their tongues.

b. The son submits his own blood sample to a local genotyping lab, and they establish that he is heterozygous for the trait.

c. His father's mother cannot roll her tongue.

d. The son's own daughter cannot roll her tongue.

e. The son's sister is a tongue roller.

Answer: c - His father's mother cannot roll her tongue.

Explanation: His father must have inherited the trait from his grandfather. This means that, the genotype of the grandfather can either be Homozygous (RR) or heterozygous (Rr) for the trait while the genotype of the grandmother can only be rr since she can't roll her tongue.

Crossing RR with rr (possible genotypes of his paternal grandparents), the probability that his father will be heterozygous for the trait is ½.

Crossing Rr with rr (another possible genotypes of his paternal grandparents), the probability that his father will be heterozygous for the trait is 1.

In either way, one can establish the heterozygosity of his father only if his father's mother cannot roll her tongue.