With hypomorphic mutations, how do we know if the gene is positive or negative (white gene produces color or inhibits color)?
Hypomorphic alleles of positive and negative genes will give you opposite phenotypes. There were 2 hypotheses about the function of the wildtype white gene, given the phenotype: namely, white promotes color (acts positively), or white inhibits color (acts negatively). In the first case, white eyes must arise from a loss-of-function mutation, while in the latter, white eyes must arise from a gain-of-function mutation (loss of an inhibitor of color would make the eyes more red, not less). The dosage experiments discussed above will distinguish between the two. Additionally, the white eye mutation is recessive, so that gives you a pretty big clue!
