THE MECHANISM OF HEREDITY 51 



discuss here only the broader relationships of Mendelian 

 heredity to the behavior of the chromosomes, since this 

 phase must be emphasized as a basis for correlating the 

 facts from Nature's experiments on inbreeding and out- 

 breeding with the results from the experiments made 

 by man. 



The Mendelian method of studying heredity consists 

 essentially in crossing forms which differ by well-defined 

 characteristics and in following the distribution of these 

 characteristics separately and quantitatively in the suc- 

 ceeding generations. If a wheat with a long lax head or 

 spike is crossed with one having a short dense spike the F, 

 (first filial) generation bears intermediate spikes. The 

 Fi generation, self-fertilized, however, yields all three 

 types — long, intermediate and short spikes — in the F2 

 generation ; and in large numbers these types bear a con- 

 stant ratio to each other in the proportion 1 long spike : 

 2 intermediate spikes : 1 short spike. Nor is this all. The 

 long-spiked plants all breed true to long spikes, the short- 

 spiked plants all breed true to short spikes, while tlv 

 plants bearing intermediate spikes again produce the 

 ratio exhibited by the F^ generation. Diagrammatically 

 the result of the cross is as follows : 



Pj Long spikes x Short spikes 



I 

 Fj Intermediate spikes 



^-^ II 



F„ Long spikes Intermediate spikes Short spikes 



I ^ W ^^ I 



Fg Long spikes Long spikes Inter- Short spikes Short spikes 



mediate 

 spikes 



