Human hair color is a classic, if oversimplified, example of recessive epistasis. Red hair is caused by a recessive allele r. However, if an individual has a dominant R allele, they may have either brown or blonde hair depending on whether they have a dominant B allele (which causes brown hair), or are homozygous for a recessive b allele (which causes blonde hair). The B and R loci are on located on different chromosomes. If a couple with genotypes Bb; Rr and bb; rr have children, what hair colors and in what proportions are expected among their children?

According to the given information, the recessive allele "r" gives red color to hair but is epistatic to alleles B and b. Therefore, the genotype with two copies of the "r" allele would have red-colored hair. The genotypes with at least one copy of "B" and “R" alleles each would have brown hair while the "R" allele would give blond hair in presence of allele "b".

Therefore, a cross between BbRr and bbrr would produce progeny in following phenotype ratio= 1 Brown: 1 Blonde: 2 Red