38 MENDELISM CHAP. 



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, contain 

 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 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 addi- 

 tional modifying factors. Singleness, in fact, underlies all 

 these combs, and if we write their zygotic constitution in 

 full we must denote a walnut as RRPPSS, a rose as 

 RRppSS, a pea as rrPPSS, and a single as rrppSS. The 

 crossing of rose with pea results in a reshuffling of the 

 factors concerned, and in accordance with the principle 

 of segregation some zygotes are formed in which neither 

 of the modifying factors R and P are present, and the 

 single character can then become manifest. 



