Answer
Ruby and white eyes are mutations. When crossbreeding 2 subjects that are defective in different genes their offspring will have a bare minimum of 1 normal gene at each locus. This means that offspring with brick red eyes are likely, as both of the parents carry mutations on different genes.
Work Step by Step
All the progeny flies inherited one functional copy of the white gene and one functional copy of the ruby red gene. As both the received chromosomes were normal, the offspring flies did not carry the mutation and instead have brick red eyes because the mutated genes are recessive to the wild allele, that being the brick red eyes.
