PLANT BREEDING AND EVOLUTION 1ST 



exactly resembled the original parents (8S and ww). These 

 two strains, when inbred, proved to be pure strains, as indicated 

 by the great-grandchildren (SS and ww). The grandchildren 

 also carried a hybrid strain (Sw), as indicated above, which by 

 inbreeding split up into pure strains (SS and ww) and into a 

 hybrid strain (Sw). 



This is clearly shown in the great-grandchildren offspring of 

 $w in the figure. Moreover, these pure and hybrid grandchildren 

 occurred in a definite mathematical ratio of |- pure SS, J pure ww, 

 and J- hybrid Sw. From this it follows that, on the average, 

 out of every four grandchildren one would be pure SS, one 

 would be pure ww, and two would be hybrid Sw. This ratio, 

 established by Mendel in experiments with peas, has been found 

 to hold true for a large number of plant and animal characters. 

 In the case of other characters there is still much doubt concern- 

 ing the applicability of Mendel's ratio. The fact remains, how- 

 ever, that Mendel's method of working with single characters, 

 and his demonstration of a possible law for the combination of 

 these characters in the offspring, has proved to be of immense 

 theoretical and practical value to experimenters in heredity and 

 plant breeding. These practical applications of Mendel's theories 

 will be discussed at the end of the chapter. 



Gametic purity. Mendel's interpretation of such results of 

 crossing as we have just described will be made clear by Fig. 98 

 and the following brief explanation. Mendel assumed that the 

 original parents (SS and ww) produced male and female gametes, 

 each of which carried but one of the characters S and w. In other 

 words, the two characters which united at the time of fertili- 

 zation, and which appeared to occur together in the seeds and 

 adult plants, always separated in the gametes, so that the 

 gametes (1) were pure and not mixed in composition, so far as the 

 two characters were concerned. This is known as the law of 

 gametic purity, which is perhaps the most important of Mendel's 

 theoretical conclusions. 



Since, moreover, the male gametes are borne by the pollen 

 grains and the female gametes are formed in the embryo sacs 

 of the ovules, chance pollination of the stigmas of a series of 



