Theory of Plant Breeding 



will be seen, three to one. The explanation as given 

 by Mendel was very simple and effective. 



We will suppose that, when '^yellow" and '^ green" 

 elements occur together in one germ cell, the progeny 

 must be yellow, that is, we will suppose that yellow 

 dominates over green, or is a dominant, whilst yellow is 

 recessive, using the original terms. 



Then if we suppose that the first generation is all 

 mixed, we may represent them as (G + Y), (G + Y). If 

 those first generation hybrids are crossed we may take 

 their constitution as follows : GG, GY, YG, YY. 



The GG are pure green, and the YY are pure yellow, 

 but both GY and YG will appear yellow because the 

 yellow dominates the green, so that the proportion of 

 three yellow to one green is maintained because yellow 

 dominates over green. 



Both GG and YY are pure green and yellow respec- 

 tively, and should breed true for any number of genera- 

 tions so far as this one character is concerned and if not 

 influenced by change in the environments. 



It is clear that this theory explains many cases of 

 heredity ^' skipping a generation," and of reversion to an 

 earlier type. So the theory is of very great importance 

 to all hybridisers and those seeking to fix a new and 

 valuable character. As Mr. Bateson, the great authority 

 on this question, has said, it may enable a practical 

 breeder to fix his type in four instead of in ten or 

 twelve generations. 



Mendel's theory has been tested in a very large 

 number of cases. It has been found to apply to certain 

 characters of peas, of poultry (colour and comb), of 

 horses (colour), of rabbits (colour), of mice (" waltzing " 

 and colour), and especially it was found to hold in no less 

 than 4548 pairs of specific characters in orchid hybrids. 



In this last experiment conducted by Hurst,^^ the re- 



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