The pedigree represents how a certain sex-linked recessive trait is inherited. Which is the most likely genotype of the grandfather? XdY XDXd XDY XdXd

To determine the genotype of the grandfather from the pedigree displaying a sex-linked recessive trait, we need to analyze the inheritance patterns associated with sex-linked traits, particularly considering that males have one X and one Y chromosome (XY) and females have two X chromosomes (XX).

1. ### Break Down the Problem

• Understand the possible genotypes for the trait in males and females.
• Identify how the trait is passed down from generation to generation, particularly from the grandfather to his offspring.

2. ### Relevant Concepts

• A sex-linked recessive trait will manifest in males with one copy of the recessive allele (X^dY) because they have only one X chromosome.
• For females to express the trait, they must have two copies of the recessive allele (X^dX^d).

3. ### Analysis and Detail

• Since the grandfather is male, his genotype must include a Y chromosome.
• His possible X chromosome alleles are X^D (dominant) or X^d (recessive).
• If the grandfather had the genotype X^dY, he would express the recessive trait. If he expressed no symptoms of the trait, he must have at least one dominant allele (X^D).
• Since the trait is recessive, if a male child inherits a X^d from the grandfather, he could potentially express the recessive trait depending on the mother’s genotype.

4. ### Verify and Summarize

• If any of the son's children expressed the recessive trait (typically seen affected males), it suggests that the grandfather carried the recessive allele.

Given this breakdown, we can analyze the options:

• X^dY: This would mean the grandfather expresses the trait.
• X^DX^d: Impossible for a grandfather since he is male.
• X^DY: The grandfather does not express the trait and has the dominant allele.
• X^dX^d: Also impossible; males cannot have two X chromosomes.

Final Answer

Based on the analysis and the characteristics of sex-linked recessive inheritance, the most likely genotype of the grandfather is:

X^DY (as he does not express the recessive trait).