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 BPpb = purple. 



White pea-flower Pink pea-flower 



Bp X Pb 



purple ancestral form 

 BPpb 



PPBb, PpBB, 



PPBB, PPBb, PpBB, PpBb PPbbPpbb ppBB, ppBb 



PPBb, PpBB, PpBb Ppbb ppBb, ppbb 



Purple 9 Pink 3 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 : 



PINK (homozygous) X WHITE 



1. WPftl X BBpp 



2. PPbb X Bbpp 



3. PPbb X bbpp 



PURPLE 



100% 



50% 



PINK 



50% 

 100% 



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. 



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