286 



THE THEORY OF THE GENE 



the stripe was narrower. Selection had in some way- 

 changed the width of the stripe. So far there is nothing 

 in the results to show that this change may not have been 

 due to the sorting out by selection of two sets of factors 

 that determine the width of the dorsal stripe. Castle 

 argued, however, that he was dealing with the effect of a 



Fig. 151. 

 Four types of hooded rats. (After Castle.) 



single gene, because when the striped rats are crossed to 

 a rat with uniform coat (all black or all gray) and the 

 hybrid (F 2 ) rats are inbred, their offspring give three 

 uniform to one spotted coat. This Mendelian ratio does 

 show, in fact, that a spotted coat is due to a recessive 

 gene, but it does not show that the effects of this gene 

 may not also be influenced by other genetic factors that 

 determine the width of the stripe, and this is really the 

 question at issue. 



