What does the purple eye color mutant allele do in a fruit fly drosphilla?

Answer 1

Sometimes, the genes that encode for one phenotype (or measurable product) can have multiple endpoints. One common example of this is in white cats. A gene in cats that suppresses coat color also suppresses hearing, resulting in all white cats with blue eyes who are deaf.

In drosophila, or fruit flies, a series of eye color mutations have been described in the literature. In the case of the purple mutation (pr), these fruit flies have purple eyes as a result of the suppression of 6-pyruvoyl tetrahydropterin (PTP) synthase - a chemical necessary for making pigment. However, PTP synthase is also important for making another protein, biopterin, which contributes to making lots of important chemicals in the brain, most important neurotransmitters. Due to the fruit flies not making biopterin, they can't make neurotransmitters, and they tend to die.