The D blood group system of the horse1

Three new horse blood group factors designated E₂, E' and Y were described. Evidence was presented that these three factors as well as the factors D, J and E₁ known before, belong to the D system established by Stormont & Suzuki (1964). That system was thus extended from being a two factor, three allele system, to the most complex system known in the horse so far. It was demonstrated that the D system is closed through the factors E₂ and E'. However a few exceptions to that rule were encountered. A minimum of 10 alleles, two of which were extremely rare seemed to control the system. Frequencies of the factors and the genes of the system were given for the Swedish Trotter breed and the North-Swedish Horse breed.