268 CONCLUDING CHAPTER 



the heterozygote AB . ab produces in equal numbers 

 the germ-cells AB, Ab, aB, and ab. Among the combina- 

 tions of these germ-cells which are represented by the 

 various offspring of the heterozygote there must appear 

 Ab . Ab and aB . aB novel types which are pure in 

 constitution, and which may form the starting-points 

 for new strains or races. 



Upon this fact depends the enormous importance of 

 Mendel's law in the breeding of new and useful types 

 of animals and plants. When it is remembered that in 

 wheat, for example, resistance and non-resistance to 

 the attacks of disease, earlines^ and lateness of 

 ripening, good and bad milling quality, are all pairs of 

 Mendelian allelomorphs, and that it is now possible to 

 take a different example of these qualities from each 

 of three different strains, and to combine them together 

 in a single new variety with perfect certainty and in 

 four generations, it does not require much imagination 

 to foresee that every department of the animal and 

 plant breeding industries must sooner or later benefit 

 enormously from Mendel's discovery. 



So far we have only been dealing with the very simplest 

 of Mendelian phenomena, leading to the arithmetical 

 addition and subtraction of definite visible characters. 

 Other kinds of allelomorphs also exist which undergo 

 a similar process of segregation during gamete forma- 

 tion, following Mendel's law in a perfect manner ; but 

 which may remain entirely invisible and unsuspected 

 so long as certain other allelomorphs, belonging to 

 quite distinct pairs, are excluded from the zygotes in 

 which these invisible factors are concealed. When 



