Final answer:
The white eye trait in the question, which doesn't appear in F1 offspring with red eyes, is recessive. This is because it is masked by the dominant red-eye allele in the genetics of fruit fly (Drosophila) X-linked inheritance patterns.
Step-by-step explanation:
If all F1 offspring have red eyes while the mutant white eye trait doesn't appear, it suggests that the white eye trait is recessive. In genetics, a trait is considered to be recessive when it doesn't manifest in the presence of a dominant allele. For instance, in an X-linked cross involving Drosophila (fruit flies), the red eye color is dominant over white. Therefore, if all F1 generation members exhibit red eyes, it means they have at least one red-eye allele (which is dominant), while the white-eye allele is recessive and masked.
For example, when a P male (white-eyed) is crossed with a P female (homozygous red-eyed), all male F1 offspring inherit the Y chromosome and a single X chromosome carrying the red-eye allele (XWY), and all female F1 offspring are heterozygous, inheriting one white-eye and one red-eye allele (XWXw). Hence, the expression of red eyes in all F1 offspring indicates that the red-eye trait is dominant while the white-eye trait is recessive.