In horse color, two major loci that dictate animals’ base coat color are the Extension (E) locus and the restriction (A) locus.
When horses have the at least one copy of both the E and A alleles ,they are referred to as bay phenotype (reddish brown body with black lower legs, mane and tail). If they have the E_aa genotype, they are black phenotype. However, if a horse has the ee__ genotype, it does not have black on its body and is called chestnut phenotype (or sorrel in Quarter Horses).
Additionally, a dominant paint pattern referred to as tobiano paint phenotype (white spots that cross the animal’s back) is determined at the T locus (T_ = tobiano paint, tt = non-paint). The T and E loci are located on the same chromosome 10 cM apart.
For the questions below, consider a bay tobiano stallion that was out of a black tobinao sire and a chestnut mare.