42 APPLICATION OF PRINCIPLES OF HEREDITY TO BREEDING. 



Table VI. — Types and their percentages in the descendants of YySs for several genera- 

 tions ivith cross fertilization and continued selection to type YYSS. Regarding YySs 

 as the Jirst generation, we have: 



Types. 



Generations. 



2. 



3. 



4 



5. 



6. 



P. ct. 



P. ct. 



P. ct. 



P. ct. 



P. ct. 



6. 25 



19.8 



31. G 



41.0 



48.2 



12.50 



19.8 



21.1 



20.5 



19.3 



6. 25 



4.9 



3.0 



2.6 



1.9 



12.50 



19.8 



21.1' 



20.5 



19.3 



25. 00 



19.8 



14.0 



10.2 



7.7 



12.50 



4.9 



2.3 



1.3 



0.8 



6. 25 



4.9 



3.5 



2.6 



1.9 



12.50 



4.9 



2.3 



1.3 



0.8 



6.25 



1.2 



0.4 



0.2 



0.1 



YYSS 

 YYSs. 

 YYss. 

 YySS.. 

 YySs.. 

 ^Yyss... 



yyss... 

 yySs.... 



yyss 



The type YYSS gradually increases until in the sixth generation 

 it constitutes 48 per cent of the crop. This type, together with types 

 YYSs, YySS, and YySs, all of which appear to be the same as 



YYSS, constitutes 

 /^'erce/y/a^e of eac/? rype /n eac/?generc^^^ 94 5 ^^^^ 

 ^ared/^y/er^gf/^ of verf.ca/ /me cuf off l>y curves, ^i^^h generation. 



' I f 1 1 1 1 1 1 1 Thus in cross-ferti- 



^ lized crops mass se- 



lection to a given 

 type gradually es- 

 tablishes that type, 

 not so rapidly, how- 

 ever, as it does in 

 self -fertilized species, 

 after hybridization. 



Figure 3 shows the 

 same thing graplii- 

 cally for ten genera- 

 tions. In this figure 

 the space between 

 the top curved Une 

 and the horizontal 

 line at the top of the 

 diagram shows the 

 proportion of the 

 type Y YSS present 

 from generation to 

 generation. It is 

 seen that in the sec- 

 ond generation only a small proportion (one-sixteenth) of type Y YSS 

 is present. In generation six, almost exactly half the population is 

 YYSS, while in generation ten this type constitutes about 65 per 

 cent of the whole. The proportion of type YYSs present from 



1G5 



2^ 3 



6enerat/ons. 



Fig. 3.— Graphic illustration of the effect of mass selection in cross- 

 fertilized species. Selection for type Y YSS; types so arranged as 

 to bring together those that resemble each other. The proportion of 

 type Y YSS from generation to generation is shown by the space 

 between the top horizontal line and the upper curved line. It is 

 seen to increase from generation to generation. The proportion of 

 type YYSs from generation to generation is indicated by the ver- 

 tical width of the space })ctween the two upper curved lines. 

 Similarly, the space beginning opposite each type formula indicates 

 the proportion of that type present from generation to generation. 



