Which allele combination represents a female who is heterozygous for a sex-linked trait?

• A. X^A X^a
• B. X^A X^A
• C. X^A Y
• D. X^a Y

Answer: (A)

Verified step by step guidance

1. Understand that sex-linked traits are typically located on the X chromosome, and females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
2. Recognize that being heterozygous means having two different alleles for a particular gene. For a sex-linked trait on the X chromosome, this means the female has one dominant allele and one recessive allele on her two X chromosomes.
3. Identify the notation: \(X^A\) represents the X chromosome with the dominant allele, and \(X^a\) represents the X chromosome with the recessive allele.
4. Since the question asks for a female heterozygous for the trait, look for the combination with two X chromosomes carrying different alleles: \(X^A X^a\).
5. Confirm that other options either represent homozygous females (\(X^A X^A\)), males with dominant allele (\(X^A Y\)), or males with recessive allele (\(X^a Y\)), which do not fit the criteria of a heterozygous female.