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.

Answer: (E)

Verified step by step guidance

1. Understand the genetic basis of the widow's peak trait: Widow's peak is controlled by a dominant allele (W), while the absence of a widow's peak is controlled by a recessive allele (w).
2. Identify the genotypes of the individuals involved: The father has a widow's peak, so his genotype could be either homozygous dominant (WW) or heterozygous (Ww). The mother lacks a widow's peak, so her genotype must be homozygous recessive (ww).
3. Analyze the son's genotype: Since the son has a widow's peak, he must have at least one dominant allele (W). Given that his mother is homozygous recessive (ww), the son must have inherited a recessive allele (w) from her. Therefore, the son's genotype is heterozygous (Ww).
4. Consider the implications of the son's genotype: If the son is heterozygous (Ww), he must have inherited the dominant allele (W) from his father. This means the father must have at least one dominant allele (W), but it does not confirm whether he is homozygous (WW) or heterozygous (Ww).
5. Determine the significance of the father's mother's phenotype: If the father's mother lacks a widow's peak, her genotype must be homozygous recessive (ww). This would mean the father must have inherited a recessive allele (w) from her, confirming that he is heterozygous (Ww).