In fruit flies, the gene for eye color is located on the X chromosome and is sex-linked. Ruby eyes are a recessive trait, which means that a female fly must inherit two copies of the ruby-eye allele (one from each parent) to express the phenotype, while a male fly only needs one copy.
In a cross between a ruby-eyed female fruit fly (X^rX^r) and a wild-type eyed male (X^RY), all of the female offspring will inherit one X chromosome from the mother (X^r or X^R) and one X chromosome from the father (X^R).
If the female offspring inherit an X^R chromosome from the father, they will be wild-type eyed, regardless of the allele inherited from the mother.
However, if the female offspring inherit an X^r chromosome from the father, they will only have ruby eyes if they also inherit an X^r chromosome from the mother. The probability of inheriting an X^r chromosome from the mother is 1/2.
Therefore, 50% of the female offspring will inherit an X^r chromosome from the mother, and of these, 100% will have ruby eyes. So, 50% of the female offspring will have ruby eyes.
In summary, the percentage of the female offspring that will have ruby eyes is 50%.