92 Extracted Whites [CH. 



pollen-shapes among them will be the subject of further 

 consideration (Chap. ix). 



White (long-pollened) x White (round-pollened) 

 CrB cRb 



F^ Purple 



CcRrBb 



F. 2 . . . 2 7 purple : 9 red : 28 whites 



CRB CRb of various compositions, but 



, none containing both Cand R 



36 : 28 



9 : 7 



When these facts are made out we have no longer any 

 difficulty in understanding how it can be that various results 

 may follow the union of two white plants which would 

 breed true to whiteness in perpetuity if left to self-fertili- 

 sation. For example, white x white will always give only 

 whites unless one of the complementary factors C and R is 

 present in each parent. Whites of the form Crb and cRb 

 when crossed together will give reds only, with 9 reds to 

 7 whites in F y Whites of the form CrB and cRB crossed 

 together will give purple in F^ and in F 9 purples : 

 7 whites, no reds being present in such a family because 

 the blue factor B occurs on each side of the parentage. 



Extracted Whites. 



We have so far represented each type of white used for 

 a parent as if it were homozygous for those factors, but of 

 course it may be heterozygous for one or more of them, in 

 which case F l will contain a mixture of types. For example, 

 CCrrBb x ccRRbb will give in F^ purples and reds in equal 

 numbers. Ccrrbb x ccRrbb will give in F l a family consist- 

 ing of 3 whites : i red. Various other combinations are 

 possible and most of them have now been met with in the 

 course of experiment. 



The fact that the extracted whites, that is to say, those 

 which appear in F^ have not in all cases the same pro- 

 perties as the original parental types is readily intelligible. 

 In the early stages of the research it seemed strange that 

 whereas the original long-pollened white crossed with the 

 round-pollened white gave a coloured result, it was possible 

 and indeed more usual to find whites exclusively produced 

 by the cross of two extracted F z whites, long-pollened 



