 The functional, dominant allele (or alleles) of the agouti gene (labeled "A") enable the horse to restrict black pigment to certain parts of the coat, notably the legs, mane and tail, allowing the underlying red to show through, resulting in bay coloring.Without this gene (homozygous recessive condition "aa"), any black pigment present is unrestricted, resulting in a uniformly black coat.Thus a black horse has at least one copy of the functional, dominant "E" allele and two copies of the non-functional, recessive "a" allele.