However, it has already been mentioned that beardlessness in wheat is dominant to beard and that the absence of horns in cattle is dominant to their presence, that is, the progeny of hornless by horned cattle are without horns except for occasional traces of imperfect horns. Facts like these would seem at first sight to contradict the assertion just made that presence is dominant to absence, but it is fairly well established that in such cases one is not dealing with true absences but with suppressions. The polled breeds of cattle, for example, are hornless not because of the absence of determiners for horns but because of the presence of an additional inhibiting factor which prevents these determiners from functioning. The horned breeds are without this inhibitor. When horned and hornless individuals are crossed the presence of the inhibitor from one line of ancestry is sufficient to suppress the development of horns in the progeny. A similar explanation would, of course, apply to beardlessness in wheat. In writing double-lettered formulæ to denote the determiners of characters in hybrids the condition is represented merely by the capital and small letter. Thus Rr indicates that red is dominant to its absence. =Additional Terminology.--=In pure breeds where the determiners are alike as BB in black or bb in albino guinea-pigs, the individual is said to be a _homozygote_ (like things united) with reference to that character, while in those in which the determiners are unlike, as Bb, the individual is termed a _heterozygote_ (unlike things united) with reference to the character. Or to use the adjective forms, a pure black guinea-pig is homozygous for black pigment, an albino guinea-pig is homozygous for absence of pigment, while a cross between the two is heterozygous for pigment. Also, where the determiner of a given character is present in double quantity, that is, from both lines of ancestry, the individual is said to be _duplex_, where represented in only the single form as in heterozygous individuals, _simplex_, and where the determiner is absent entirely, _nulliplex_, with reference to the character in question. Thus black guinea-pigs of formula BB are duplex with regard to the determiner for black color, individuals of formula Bb are simplex with reference to this determiner, and those of formula bb are nulliplex. A heterozygote in which dominance prevails can be identified with certainty by breeding to a known recessive and noting the kind of offspring produced. If the individual was really a heterozygote, approximately fifty per cent. of the offspring should be of the recessive type.