In rabbits, the color of body fat is controlled by a single gene with two alleles, designated Y and y. The outcome of this trait is affected by the diet of the rabbit. When raised on a standard vegetarian diet, the dominant Y allele confers white body fat, and the y allele confers yellow body fat. However, when raised on a xanthophyll-free diet, a homozygote yy rabbit has white body fat. If a heterozygous rabbit is crossed to a rabbit with yellow body fat and the offspring are raised on a standard vegetarian diet, what are the proportions of offspring with white and yellow body fat? How do the proportions change if the offspring are raised on a xanthophyll-free diet?