34 



MENDELISM 



CHAP. 



the possible combinations are represented and in 

 their proper proportions. Fig. 5 shows the result 

 of applying this method to our series RP, Rp, rP^ rp, 

 and the 16 squares represent the different kinds of 

 zygotes formed and the proportions in which they 

 occur. As the figure shows, 9 zygotes contain both 

 R and P, having a double or a single dose of either 



or both of these 

 factors. Such 

 birds must be all 

 walnut combed. 

 Three out of the 

 1 6 zygotes con- 

 tain R but not P, 

 and these must be 

 rose-combed birds. 

 Three, again, con- 

 tain P but not R, 

 and must be pea- 

 combed birds. 

 Finally one out 

 of the 16 contains 

 neither R nor P. 

 It cannot be rose 

 — it cannot be pea. It must, therefore, be some- 

 thing else. As a matter of fact it is single. Why 

 it should be single and not something else follows from 

 what we already know about the behaviour of these 

 various forms of comb. For rose is dominant to 

 single ; therefore on the Presence and Absence theory 

 a rose is a single plus a factor which turns the single 

 into a rose. If we could remove the " rose " factor 

 from a rose-combed bird the underlying single would 



Fig. s. 



Diagram to illustrate the nature of the F2 generation 

 from the cross of rose comb X pea comb. 



