In parakeets, two genes, B and Y, contribute to two different pigment-producing biosynthetic pathways that produce either blue or yellow pigment. The wild-type parakeets have feathers that are green with a mixture of blue and yellow. They are also found to have blue feathers (due to the absence of yellow pigment), yellow feathers (due to the lack of blue pigment), and white feathers (the absence of both pigments). Using this information, determine the genotype of pure-breeding yellow parakeets.