A man has a pointed frontal hairline known as a widow's peak. This trait is thought to be controlled by a dominant allele. His wife lacks a widow's peak, and their son has a widow's peak. (W: widow's peak; w: no widow's peak). The son is curious about whether his father is homozygous or heterozygous for the widow's peak trait. Which of the following facts would allow him to know?

• A. Both his paternal grandfather and his paternal grandmother have widow's peaks.
• B. The son's sister has a widow's peak.
• C. The son's own daughter lacks a widow's peak.
• D. The son submits his own blood sample to a local genotyping lab, and it establishes that he is heterozygous for the trait.
• E. His father's mother lacks a widow's peak.