598 



Scientific Proceedings, Royal Dublin Society. 



factors could be defined ; for if E be an extension of the melanic pigment, and 

 B a deepening, why should the first of the above constitutions, with a double 

 dose of E, be only agouti-black, while the third, with only a single dose, is 

 full black? 



The second remark is : If the factors D and E are coupled — that is, if D 

 cannot enter a gamete unless accompanied by E — how is the second dose of 

 D to enter a constitution heterozygous for E ? 



We can compare the two assumptions if we set down side by side all the 

 possible constitutions containing X' on the one hand, and the corresponding 

 constitutions containing D coupled with E on tlie other ; and we can make 

 the comparison clearer by surrounding the coupled factors with parallel lines, 

 thus : — 



X' X' X' X' X X X X X X XX X X X X X X 



Y Y Y y y y YY Y y y y YY 11 y y y 



AA 



black. 



A a 



black. 



D\ d 



e\ E 



I 



a a AA A a a a 



black, agouti-black, agouti-black, black. 



AA 



black. 



d 

 e 



A a a a 

 black. black. 



On examining the above scheme we see that, since the coupled factors 

 act really as a single factor, the most essential difference between the two 

 assumptions is that one requires two recessives while the other requires three. 

 If it could be determined whether there are two or three recessives operating 

 in the case, it would be determined at the same time which of the two 

 assumptions is the more reasonable. 



But the question as to how the colours dealt with so far and several others 

 not yet mentioned are constituted can be carried still farther from information 

 supplied by Professor Punnett when he compares what he calls the Hack 

 series — the one so far dealt with here — with another series which he calls the 

 chocolate. When considering the difficulty of identifying the functions of 

 the several factors, we pointed out that other factors might be operating at 

 the same time. And this is really the case. 



Professor Punnett's tabular comparison of the two series shows that there 

 is a factor " for black " in agouti, black, yellow, and tortoise, while its recessive 

 is in the chocolate series, i.e. in cinnamon, chocolate, dilute cinnamon, and 

 orange. He symbolizes these factors by B and h ; but, since it is safer to use 

 " unknown " symbols, we shall call them Z and z. These factors — X' being 

 inoperative — and the colours to which they belong now combine to form 

 a set in which there are three pairs of factors operating and eight colours 

 produced. We shall set down first of all the names given to the colours by 



