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 t5^ical example is the case of the inheritance of horns in 

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



10 



Fig. 19. 

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

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

 in the two sexes. 



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

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

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

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

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



