34 



MENDELISM 



CHAP. 



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 16 zygotes contain 



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 1 6 contains 

 neither R nor P. 

 It cannot be rose 

 it cannot be 

 pea. It must, 

 therefore, be some- 

 thing else. As a 

 of 



FIG. 5. 



Diagram to illustrate the nature of the F 2 generation 

 from the cross of rose comb x pea comb. 



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 

 come into view. Similarly a pea comb is a single plus 

 a factor which turns the single into a pea, and a walnut 

 is a single which possesses two additional modifying 

 factors. Singleness, in fact, underlies all these combs, 



