Question

You are studying two genes that interact to determine coloration in the following way: White --> Yellow --> Green where the protein product of gene A catalyzes the first reaction and the protein product of gene B catalyzes the second. What phenotypic ratio is expected from the following cross: Aa:Bb x aa:Bb? Alleles A and B are completely dominant and both are independently assorting.

Answer 1

In 8 out of 16 cases where genotype is aa__, we will see no production of gene A's protein for catalysing White to Yellow transition, so we will be at the basal colour level White
In all other instances of Aa__ and AA__, there are 2 cases where the genotype of B allele is bb which means no gene b's protein for yellow to green transition, so we will have 2 such yellow coloured products, in the other 6 instances, we have AA__ and Aa__ combined with __Bb or __BB and hence since both functioning genes produce colour changing proteins, we get:
White:Yellow:Green :: 8:2:6