Question

In fruit flies, one genotype for red eyes is written X R X R . The R stands for the dominant allele for red eyes. What does the X stand for

Answer 1

If R is dominant, then the other trait must be recessive.
Another way to prove this is using the fact that recessive traits never show. In the fruit fly, the recessive trait is not shown.
Hope this helps!

Answer 2

The most appropriate answer would be the female sex chromosome.

X and Y are the sex chromosomes present in fruit flies. X is the female sex chromosome and Y is the male sex chromosome. However, the ratio of the X chromosome to autosomal chromosomes determines the gender of a fruit fly.

Y chromosome does not influence the gender of the fruit fly however, it carries genes responsible for other traits.

Similarly, X chromosome also carries genes which code for different traits. The alleles of genes present on the X chromosome are written as X_X_.

Thus, in XRXR, X represents the female sex chromosome.