CHAPTER XV 



some new peinciples of breeding based on 

 Mendel's theory 



How tlie theory of heredity of the future will differ 

 from that of the past can, perhaps, be most easily 

 m'ade clear by describing an experiment as to the 

 result of which the expectation based on the Mendelian 

 theory and that based on the older notions which 

 it is likely to supplant are diametrically opposed to 

 one another. 



The reader will not need to be reminded that the 

 result of crossing a yellow-seeded with a green -seeded 

 pea is a yellow, which produces a second hybrid 

 generation consisting of 75 per cent, yellow and 

 25 per cent, green. There is this proviso, however, 

 that this is only known to be true when the green 

 and yellow, with which the cross is made, belong to 

 pure green and pure yellow strains. But this should 

 make no difference according to the Mendelian theory : 

 a yellow, provided its zygotic formula is YY, and a 

 green, provided its zygotic formula is GG, should, 

 whatever their ancestry, give rise to yellows which 

 will produce 75 per cent, yellows and 25 per cent, 

 greens, when self-fertilised. The expectation based 

 on the Mendelian theory is in violent opposition to 

 what the old notions of heredity would lead us to 



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