R. C. PUNXKTT 45 



Discussion. 



From the preceding ai-coiint it is clcMr that all the fiict.s observed 

 in this series of experiments can hv iiiterjuetcd mi the " Presence 

 and Absence" hypothesis if it be supposed that complete coupling 

 obtains between the factors D and E. There is howevei- another- 

 hypothesis —that of "Multiple Allelomorphs" — which has been put 

 forward in connection with this and similar cases'. The difference 

 between the two explanations may best be brought out by considering 

 the instance used by Sturtcvant (10) in a recent discussion of the 

 problem. It was jiointed out some years ago by Hurst and by Castle 

 that albinism behaves as a simple recessive to self-colour. Both of 

 these observers have further shewn that the Himalayan pattern also 

 behaves as a simple recessive to self-cdlnur. Castle's experiments 

 again proved that the Himala^'an pattern is a simple doun'nant to 

 albinism. Nevertheless Himalayans do not appear in F.. families 

 from self-colour x albino. Sturtevant has pointed out that the 

 genetic behaviour of these colour varieties is covered by either of 

 two hypotheses. 



A. On the " Presence and Absence " hypothesis we must suppose 

 that two factors are concerned, viz. C, a colour producer 

 which is present in the self-coloured and in the Himalayan 

 rabbit, and S, a factor which turns the Himalayan into 

 self-colour. Zygotically therefore the three forms may be 

 represented thus : 



Self-coloured ... CCSS, 

 Himalayan ... CCss, 



Albino CCSS. 



The albino must be supposed to lack S because no case 

 is known of a self-coloured rabbit being produced from the 

 mating Himalayan x albino. 



In order to account for the non-apjjearance of Himala3ans 

 in F„ from self-colour x albino, we must suppose that there 

 is complete coupling between C and S, so that F^ rabbits 

 of the constitution CcSs produce only gametes of the form 

 CS and cs. 



' The hypothesis would appear to have been put forward iudepeudently by Morgan (7) 

 and by Wilson (12) in 1913 (see also note on p. 46). 



