Final answer:
The unexpected result of the cross between an eyeless and normal Drosophila indicating that eyeless is a recessive trait and the normal fly was likely homozygous dominant. To express the eyeless allele, crossing two F1 offspring or back-cross with the eyeless parent could be done. Eye color in Drosophila is X-linked and different rules apply to dominance in males.
Step-by-step explanation:
While working with the fruit fly Drosophila, the researcher observed that a cross between an eyeless fly and a normal fly produced 65 offspring all with eyes. This outcome suggests that the normal fly likely carried two dominant alleles for eye production, making the trait for having eyes dominant. Since all the offspring had eyes, the eyeless trait must be recessive.
The researcher did not do anything 'wrong', but these results indicate that the eyeless fly was heterozygous for the eyeless trait (with one mutant allele and one normal allele), or there was incomplete penetrance of the mutant eyeless allele. To produce flies expressing the eyeless allele, the researcher could conduct a cross between two of the F1 offspring assuming they're heterozygous for the eyeless trait, or back-cross an F1 offspring with the original eyeless parent.
It's also important to remember that eye color in Drosophila is an X-linked trait, where males are hemizygous (having only one allele for any X-linked characteristic) and females have two alleles. This makes descriptions of dominance and recessiveness different from autosomal genes. For X-linked traits, a cross between a male expressing a recessive trait and a female homozygous dominant for that trait will result in all offspring exhibiting the dominant phenotype, with females being heterozygous and males being hemizygous for the dominant trait.