MUTATION. 



• 149 



example of the black colour in oats is a very striking one. But 

 we have tried to show that as there is no series of cases of in- 

 complete dominance, ranging from nearly complete dominance 

 down to 50% dominance, we may feel safe in assuming that if 

 two forms crossed differ in only one gene, that parent, which 

 has the dominant character, had a gene more than the other. 

 For the present we think, dominance is a good criterion for 

 presence of an additional gene. 



We think it will be necessary in this connection briefly to 

 treat of a few cases, which have been held to show how the 

 presence of a gene can be recessive to its absence, namely 

 those cases from which it is said that they show, how a charact- 

 er can be dominant in one sex and recessive in the other sex. 

 A typical example is the case of the inheritance of horns in 

 sheep, brought forward by Woods. (Fig. 19). 



{m, 



Fig. 19. 

 Diagram to illustrate the effect of different combinations of genes in- 

 fluencing horn-development if tendency to horn-formation is unequal 

 m the two sexes. 



Woods mated Dorset to Suffolk sheep. Dorset is a species of 

 domestic sheep in which both sexes are horned, and in the 

 other species both rams and ewes are horn-less. Woods obser- 

 ved that the male hybrids were horned, and that the females 

 were hornless, like the Suffolk. If a F2 generation was raised, 



