R C. PUNNKTT 227 



the absence of the other factor upon the other side of the cross. 

 Without going into detail I may say that this hypothesis proved 

 incapable of affording a complete explanation of the facts. Indeed 

 the matter turned out to be considerably more complicated than at first 

 appeared and it was only after three years work that a satisfactory 

 hypothesis was eventually built up. Nevertheless the subsequent 

 experiments will be easier to follow if at this point I set out the 

 explanation which I have been led ultimately to frame. 



The Hypothesis. 



In the set of experiments into which f^ 28 and his descendants 

 enter we are dealing essentially with three separate factors' : 



A, the "agouti" factor which turns black into agouti, and tortoise 

 into yellow. 



E, a factor for the extension of the melanic pigment which turns 

 yellow into agouti and tortoise into black. 



D, a factor of which the effect is to produce a deepening in the 

 melanic pigment^. 



The effect produced by D depends (1) upon whether this factor is 

 present in a homozygous or in a heterozygous condition, and (2) upon 

 whether the animal is homozygous or heterozygous for E. The addition 

 of one dose of D to an agouti which is homozygous for £ turns it into 

 an agouti-black, while the addition of a second dose results in a full 

 black. If however the agouti is heterozygous for E, the addition of 

 either one or two doses of D produces the same visible effect, viz. a 

 full black. The presence of D in a black makes no difference to the 

 appearance of the animal. For reasons which will appear later it is 

 probable that D does not occur in yellows or tortoises. • 



The factor D must be supposed to have been brought in by the 

 Himalayan doe ($ 7), and her zygotic constitution was probably 

 DdEEaa. The yellow buck {^ 5) with which she was crossed was 

 known to be heterozygous for A and was therefore ddeeAa. Of the 

 two animals from this mating which were subsequently bred from, 

 ? 27 was evidently ddEeAa (cf. Table I), having been formed by 



^ The factor whose presence or absence determines whether the animal shall be self- 

 coloured or Himalayan may for the present be left oat of accoont. 



^ It is possible to look at the matter from a rather different point of view and to regard 

 S as a factor which prevents the working of the agoati factor. The agoati factor is itself 

 in a sense an inhibitor of black. Consequently on this view we should have to regard 

 the factor D as an inhibitor of an inhibitor. 



