To determine which allele combination represents a female who is heterozygous for a sex-linked trait, we need to understand a few key concepts:
1. Sex Chromosomes:
- Females have two X chromosomes (XX).
- Males have one X and one Y chromosome (XY).
2. Heterozygous:
- Being heterozygous means having two different alleles for a particular trait.
Given these criteria, let's analyze each option:
- Option 1: [tex]\( X^R Y \)[/tex]
- This combination represents a male because it consists of one X chromosome and one Y chromosome.
- Option 2: [tex]\( X^{\prime} Y \)[/tex]
- This combination also represents a male because it includes one X chromosome and one Y chromosome.
- Option 3: [tex]\( X^R X^{\prime} \)[/tex]
- This combination represents a female because it consists of two X chromosomes.
- Additionally, it shows different alleles (one X with allele [tex]\( R \)[/tex] and another X with a different allele [tex]\( X^{\prime} \)[/tex]).
- Thus, this female is heterozygous for the trait.
- Option 4: [tex]\( x^R x^R \)[/tex]
- This combination represents a female because it consists of two X chromosomes.
- However, it shows the same allele ([tex]\( R \)[/tex]) on both X chromosomes.
- This female is homozygous, not heterozygous, for the trait.
Therefore, the allele combination that represents a female who is heterozygous for a sex-linked trait is:
[tex]\[ X^R X^{\prime} \][/tex]
Thus, the correct answer is option 3.
