Question

the pedigree shows a sex-linked recessive trait (r). a male in the second generation is labeled with a question mark because his condition is unknown.

which option represents a possible genotype/phenotype combination for the unknown male?

a. \\(x^ry\\), affected male
b. \\(x^rx^r\\), affected male
c. \\(x^rx^r\\), carrier male
d. \\(x^ry\\), unaffected male

Explanation:

Analyze the inheritance pattern

Using the Genotype and Phenotype knowledge points

• The trait is sex-linked recessive, denoted by allele \(r\).
• Let \(X^R\) be the dominant unaffected allele and \(X^r\) be the recessive affected allele.
• Males have one X chromosome and one Y chromosome, so their possible genotypes are \(X^R Y\) (unaffected male) or \(X^r Y\) (affected male).

Determine the genotype of the unknown male's partner

• The unknown male (labeled with a "?") mates with an unaffected female (circle).
• They have four children:

1. Unaffected male (\(X^R Y\))
2. Carrier female (\(X^R X^r\))
3. Unaffected male (\(X^R Y\))
4. Carrier female (\(X^R X^r\))

• Since the daughters are carriers (\(X^R X^r\)), they must inherit one \(X^r\) allele from one parent and one \(X^R\) allele from the other parent.
• Since the sons are unaffected (\(X^R Y\)), they must inherit their \(X^R\) allele from their mother, meaning the mother must carry at least one \(X^R\) allele.
• If the mother is unaffected and not a carrier, her genotype is \(X^R X^R\).

Determine the genotype of the unknown male

• The daughters are carriers (\(X^R X^r\)).
• Since the mother is \(X^R X^R\), she can only pass on the \(X^R\) allele to her daughters.
• Therefore, the daughters must inherit their recessive \(X^r\) allele from their father (the unknown male).
• Since the father is male, his genotype must be \(X^r Y\).
• A male with the genotype \(X^r Y\) has the recessive allele on his single X chromosome, which makes him an affected male.

Answer:

• A. \(X^r Y\), affected male (Correct answer)
• B. \(X^r X^r\), affected male
• C. \(X^R X^r\), carrier male
• D. \(X^R Y\), unaffected male