The ¥2 Generation 



The results of simple Mendelian segregation in the Fn 

 generation may be summarized as follows : 



The F2 Generation or the Progeny of Hybrid x Hybrid: 



In breeding work by crossing, therefore, if the pheno- 

 types in the Fi generation are in the ratio of 3 : 1 it is as- 

 sumed that only one pair of parental characters is present ; 

 if in the ratio of 9 : 3: : 3: 1, or in 9 : 7 or 15 : 1, two pairs of 

 characters are present; and if in the ratio of 27 : 9 : 9 : 9 : 3 

 3 : 3: 1, or 63 : 1, three pairs of characters are present. 



Conception of the Germ Cells 



The results of Mendel's experiments and those of later 

 investigators lead to the following conceptions of the germ- 

 cells : 



1. Their purity with regard to the characters they bear, and 

 their presence in equal numbers. 



2. Each contains a complete set of characters (or factors 

 that determine the characters) of the individual. 



3. The fertilized egg (zygote) is a double structure, contain- 

 ing the character derived from the male parent, also that 

 from the female. These characters may be similar 

 (homozygous) or they may be dissimilar (heterozygous). 



4. The germ-plasm of the germ-cells has a definite structure 

 (see also page 109). 



(c) — Additional Examples of Simple Mendelian 



Hybrids 



The following examples not only illustrate the appli- 

 cation of Mendelism to the production of new varieties of 

 plants and animals, but also unravel some of the perplexing 

 problems of the breeder. 



96 



