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question 12
sex - linked conditions are more common in men than in women because

1. men acquire two copies of the defective gene during fertilization
2. men need to inherit only one copy of the recessive allele for the condition to be fully expressed.
3. the sex chromosomes are more active in men than in women
4. the genes associated with the sex - linked conditions are linked to the y chromosome, which determines maleness

Explanation:

• Option 1: Men have \(XY\) sex - chromosomes. They receive one \(X\) from the mother and one \(Y\) from the father. So, they cannot acquire two copies of a gene (as they have only one \(X\) - linked gene in case of sex - linked conditions, not two), so this option is incorrect.
• Option 2: For sex - linked (usually \(X\) - linked) recessive conditions, women have \(XX\) chromosomes. They need two copies of the recessive allele (\(X^{r}X^{r}\)) to express the recessive phenotype. Men have \(XY\) chromosomes. Since there is no corresponding allele on the \(Y\) chromosome to mask the recessive allele on the \(X\) chromosome, men only need one copy of the recessive allele (\(X^{r}Y\)) to express the condition. This is correct.
• Option 3: There is no general rule that sex chromosomes are more "active" in men than in women in the context of sex - linked conditions. The expression of sex - linked genes is mainly due to the difference in the number of \(X\) chromosomes (and the lack of a homologous \(Y\) chromosome for most \(X\) - linked genes in men), not activity levels. So, this option is incorrect.
• Option 4: Most sex - linked conditions (like hemophilia, color - blindness) are \(X\) - linked. The \(Y\) chromosome has very few genes compared to the \(X\) chromosome, and most sex - linked conditions are not \(Y\) - linked. So, this option is incorrect.

Answer:

1. men need to inherit only one copy of the recessive allele for the condition to be fully expressed.