RELATING TO SECONDARY SEXUAL CHARACTERS. 67 



It may be pointed out in passing that if, as Arkell assumes, the 

 hornless races are due to the presence in them of an inhibitor for horns, 

 the results can be worked out without postulating that the inhibitor 

 is sex-linked. For example, if the hornless male and female be HHII 

 and the horned male and female HHii, the FI horned males and 

 hornless females will be HHIi. The germ-cells will be HI and Hi in 

 each sex, which, by chance meeting, as shown below, gives the results 

 obtained by Wood. Thus: 



HI v. Hi female. 



HI A Hi.. ..male. 



1H1HI+2, HIHi+1, HiHi. 



These formulas give 3 horned males, 1 hornless male, 1 horned 

 female, 3 hornless females. This formulation, while appealing appar- 

 ently to a different set of factors from those used by Arkell, is in reality 

 the same in principle, since the heterozygous condition is here repre- 

 sented by li (instead of Hh) and sex determines that the heterozygous 

 male is horned and the female hornless. 



The genetic relations of the Merino with horned males and hornless 

 females to the Dorsets, in which both sexes are horned (but in the 

 male the horns are larger), must be different from the genetic relation 

 in the other cross. There are two theoretical possibilities, viz, that a 

 different factor for horns is present that is either an allelomorph or 

 another different factor; or second, that a modifier is present in the 

 Merino that keeps down the development of the horns in the female. 

 An answer could be obtained by breeding Merinos to horned and to 

 hornless and getting F 2 from both crosses. Arkell's data is not sufficient 

 to settle the question, because his numbers are often too small, but 

 chiefly because it appears that there were two genetic types present 

 in his flock of Merinos, one of which is characterized by scurs (very 

 short horns) in the females, the other by hornlessness in the female. 

 He found in a cross between a hornless father and Merino mother 

 (that had knobs or scab-like growths) that the daughters had horns 

 or scurs and carried a determiner for horns (as subsequent generations 

 showed). On the other hand, in other cases where the Merino mother 

 was without horns, her FI daughters had no horns. In both cases the 

 FI sons had horns. Arkell cites this cross as " proving" that the knobs 

 of Merino ewes depend for their development upon two horn deter- 

 miners (H'H'). It is not at all evident that the results lead to such 

 a conclusion, as other explanations will cover the case as well. 



Arkell's mating between Dorsets and Merinos (tables ix and xvi) 

 corroborates his view "that the knob of the Merino female is repre- 

 sented in the germ-plasm by the double determiner." The 5 FI sons 

 had long horns, 3 FI daughters had horns present, and 2 had them 

 absent (table xvi). If some of the Merino mothers used were homo- 

 zygous for a factor that inhibits the development of horns in the female 



