In fruit flies, the traits for a yellow body and white eyes are always inherited together. Which conclusion can be drawn from this phenomenon?

A. The genes for the traits are adjacent in the genome.

B. Both traits are dominant.

C. The species has only one allele for each trait.

D. The traits are controlled by one gene.

I believe

C. The species has only one allele for each trait

Answer: Option C. The species has only one allele for each trait is correct.

Any trait in an organism can be determined by a single gene containing two alleles one from each parent and one allele dominant over the other. A single trait is affected by a single locus. Only one single trait can be expressed by a single gene and a gene can never encode for more than one trait. The traits that are expressed are independent of the locus of the gene or position of the gene. Without knowing the genotype it cannot be predicted that both traits are dominant or not and it may be possible that either of the trait is recessive. Therefore it can be concluded that the species has only one allele for each trait which is expressed.