MENDELISM 103 



6. THE CARDINAL, PRINCIPLE OF SEGREGATION 



The essential thing which Mendel demonstrated was 

 the fact that, in certain cases at least, the determiners 

 of heredity derived from diverse parental sources may 

 unite in a common stream of germplasm from which, 

 in subsequent generations, they may segregate out ap- 

 parently unmodified by having been intimately asso- 

 ciated with eadh other. This law of segregation, or 

 "independent assortment" as Morgan prefers to call it, 

 depends upon the conception that the individual is 

 made up of a bundle of unit characters. It may be 

 illustrated by the separate flowers picked from a garden 

 which, after being made into a nosegay, may be taken 

 apart and rearranged without in any way disturbing 

 the identity of the separate blossoms. 



The general formula of segregation that covers 

 all cases of organisms cross-bred with respect to a 

 single character, that is, monohybricfo, is given in 

 Figure 17. , 



The parents of a hybrid are usually referred to as 

 the parental generation (P). The hybrid generation 

 formed by crossing diverse characters 'in parents is 

 designated as the first filial generation (Fj). The 

 offspring of F x are F 2 , and so on. 



Incidentally this diagram hints how it is possible 

 to derive a pure strain from an impure (hybrid) source, 

 a fact of immediate interest not only to breeders of ani- 

 mals and plants but also to breeders of men. 



Such "extracted" recessives or dominants will be en- 

 tirely free of the hybrid impurity. 



