10 N. H. AGR. EXPERIMENT STATION. [Bulletin 160 



easy to suppose that there is some other factor regularly coupled 

 with maleness which has this property, though that possibility 

 cannot be absolutely excluded. The suggestion, however, that 

 the female contains something which suppresses the effect of 

 the otherwise dominant factor is consistent with the observation 

 that when these sex-limited conditions, as they are called, do 

 appear in females, they are developed to a somewhat less degree 

 than in males, just as in horned breeds of sheep the ewes have 

 horns smaller than those of the rams." 



Reviewing the evidence of other investigators in this regard 

 and combining with it our own gotten from actual breeding tests 

 with many different breeds of sheep at this Station and the 

 Station for Experimental Evolution, Davenport and myself * 

 (1912) formulated the hypothesis, that horns in sheep represented 

 a typical sex-limited character. This was based on the assump- 

 tion that there is an inhibitor to horn formation located on the 

 sex chromosome. The hypothesis further assumes that, as in 

 man so in sheep, the male is heterozygous (simplex) in sex. One 

 sex chromosome is then to be expected in the male, and sub- 

 stantially this condition has been found to hold for man by 

 Guyer f (1910). The female will then be duplex in respect to 

 sex. Consequently, since the inhibitor is sex-linked it will be 

 simplex in the male and duplex in the female. The inhibitor, 

 then (designated in Tables I and II by the letter I, its absence by 

 i) will at all times be double in the female and single in the male, 

 and, in the gametes, will always be associated with the sex chroma- 

 some, which is designated throughout by the symbol X; its 

 absence by x. There are, moreover, two classes of horn deter- 

 miners; namely, the Dorset Horn type (H), and the Rambouillet 

 type (H'). H is a vigorous, sturdy determiner, capable of pro- 

 ducing under fertile conditions a heavy somatic growth of horn. 

 H' is a weaker determiner, more easily controlled and dominated 

 by the reactionary influence of the inhibitor. In the zygote the 

 single inhibitor is. incapable of preventing the development of 

 either horn determiner, even when simplex. However, in the 

 simplex condition, since there is only one "dose" of horn in the 

 germ plasm, the somatic growth is greatly reduced and in the 

 case of the weak determiner, H', potency runs still lower. But 

 the double inhibitor is capable of checking and preventing entirely 

 somatic appearance of the single horn (Hh or H'h) but not 

 the double determiner (H H or H' H'). Again, H' H' does not 

 show equal potency in the soma, but exists in females as knob- 

 like protuberances, scabs or scurs, which I have previously 

 described as possessed by Merino ewes. In Merino males the 

 duplex determiner, since only one inhibitor is reacting against 



*Arkell, T. R., andJDavenport, C. B., "Horns in Sheep as a Typical Sex-Limited Character." Science, 

 N. S., Vol. xxxv, No. (597, pp. 37J-377. 

 f Guyer, M. F. " Accessory Chromosomes in Man," Biol. Bull, six, 21D, 231, pi. I. 



