Question

1. explain why the presence of a recessive allele on one x chromosome often has different consequences for a man than for a woman.

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Explanation:

1. Chromosome Composition: Males have one X and one Y chromosome (genotype \(XY\)), while females have two X chromosomes (\(XX\)).
2. Recessive Allele Expression: For a recessive allele on the X - chromosome:

• In males (\(XY\)), there is no second X - chromosome to carry a dominant allele that could mask the recessive allele. So, if the X - chromosome has a recessive allele, the trait associated with that recessive allele will be expressed.
• In females (\(XX\)), if one X - chromosome has a recessive allele, the other X - chromosome may carry a dominant allele. This dominant allele can mask the expression of the recessive allele, so the recessive trait is less likely to be expressed (unless both X - chromosomes carry the recessive allele, which is less common).

Answer:

The presence of a recessive allele on one X - chromosome has different consequences for men and women because men have a single X (and a Y) chromosome (\(XY\)), so a recessive allele on their only X will be expressed. Women have two X chromosomes (\(XX\)), so a recessive allele on one X can be masked by a dominant allele on the other X, and the recessive trait is expressed only if both X chromosomes carry the recessive allele.