Question

3 multiple choice 1 point
in drosophila (fruit flies) eye color is a sex-linked trait. with red eyes being dominant to white eyes. the pedigree below shows the dominant sex-linked trait.

1000 offspring, all red-eyed

based on the given pedigree what is the most likely genotype of the parent generation?
$x^r x^r$ and $x^r y$
$x^r x^r$ and $x^r y$
$x^r x^r$ and $x^r y$
$x^r x^r$ and $x^r y$

Explanation:

Step1: Recall sex-linked trait rules

For X-linked traits, males have genotype $X^?Y$, females have $X^?X^?$. Red ($X^R$) is dominant over white ($X^r$).

Step2: Analyze male parent phenotype

The male parent is white-eyed (unfilled square), so his genotype must be $X^rY$ (since males only have one X chromosome, recessive phenotype means recessive allele).

Step3: Analyze offspring phenotype

All >1000 offspring are red-eyed. For female offspring: they get one X from father ($X^r$) and one from mother. To all be red-eyed, mother must pass only $X^R$, so mother is $X^RX^R$. For male offspring: they get X from mother ($X^R$) and Y from father, so they will be $X^RY$ (red-eyed), matching the offspring phenotype.

Step4: Match to options

The parent genotypes are $X^RX^R$ (female) and $X^rY$ (male).

Answer:

$X^R X^R$ and $X^r Y$