THEORY OF RESULTS 217 



There remains one section of the theory to be 

 dealt with. This relates to what has been called 

 gametic purity. By this is meant the idea that 

 the distribution of unit-characters amongst the germ- 

 cells is discontinuous ; in other words that a germ 

 cell either contains the factor for a particular unit- 

 character or it does not ; there is no intermediate 

 condition, no half-way house. It is not possible for 

 a germ cell to contain a mixture of yellowness and 

 greenness ; that was why YG was not a permissible 

 combination of letters amongst the formulae of the 

 gametes outside the sixteen-square Table. Also, it 

 is not possible for a germ-cell to have more or less of a 

 particular character ; it has either got it or it has 

 not. That was why GG was not a permissible 

 formula for a gamete. For a zygote it is, of course. 



There has been a good deal of misunderstanding as 

 to what is meant by gametic purity, and consequently 

 a good deal of unintentionally irrelevant criticism of 

 Mendelian theory. Let me indicate what the precise 

 significance of the Mendelian teaching on this point 

 is. One of the most practically important of the 

 corollaries which follow from this doctrine of gametic 

 purity is that the extracted dominants or recessives 

 of the second hybrid generation are identical in 

 respect of a particular character with their pure 

 dominant or recessive grandparents which were 

 mated to make the cross. The form which this 

 sometimes takes in the mouths of objectors is, " The 

 extracted recessive or dominants are identical with 

 the pure dominant and recessive grandparents, 



