Final answer:
A single copy of the mutant allele X" in Drosophila males confers white eyes because they lack a second allele copy on the Y chromosome. This means that if a male inherits the X" allele, there is no dominant red-eye allele to mask the effects of the mutant allele.
Step-by-step explanation:
The white eye color in Drosophila is caused by a mutant allele called X".
In males, the X chromosome contains only one allele for any X-linked characteristic because they lack a second allele copy on the Y chromosome.
Therefore, if a male inherits the X" allele, he will have white eyes because he does not have a dominant red-eye allele to mask the effects of the mutant allele.