An Introduction to a Biology 



flower. These crossed give a purple flower, the ancestral 

 form. The experiment explained, on the basis of two sets 

 of factors, thus : — 



B = factor producing blue only in the presence of factor for 

 pink, P ; 

 hence Bp == white, and therefore BPjjh == purple. 



White pea-flower Pink pea-flower 



Bp X Pb 



purple ancestral form 

 BPph 



PPBh, PpBB, 



PPBB, PPBh, PpBB, PpBb 



PPBh, PpBB, PpBh 



Y 



Purple 9 



PPhh Pphb 

 Pphh 



Pink 3 



ppBB, ppBb 

 ppBh, pphh 



White 4 



This is a convenient way of expressing the results, but 

 it does not really signify anything as to what is actually 

 going on in the plant. If four whites really have these com- 

 binations of factors, then it may be tested by crossing the 

 whites with a homozygous pink, and theoretically the results 

 should be : — 



This has been done with a small number of plants, and 

 is being repeated on a large scale by Darbishire. Darbi- 

 shire found that in the cross (2. above), PPbb x Bbpp, 

 the ratio was about 3 : 1 and not 1:1. 



lOI 



