33 



way that one half the number of germ-cells contains the factor for 

 one character only of any pair, and the other half only the factor for 

 the opposite character. This conception, sometimes referred to as 

 " gametic purity," constitutes the essence of Mendel's law. 



All the facts obtained by Mendel in his experiments on peas, so 

 far as the seven pairs of characters already referred to were concerned, 

 pointed to the correctness of his explanation. Taking the simplest 

 case, for example, in which only two contrasted characters are 

 involved, it is easy to see that if, when the germ-cells of the hybrid 

 are formed, they separate into two equal sets, containing respectively 

 the factors for, say, yellow and green cotyledons, the only possible 

 combinations are as follows : 



$ y with $ y =i yy (/. e. pure yellow) 



H : |;} = 2yg <" h ? hM - > 



<J g „ ? g = i GG ( „ pure green) 

 The results mentioned in the foregoing paragraphs may be indicated 

 in the form of a diagram, thus : 



R X A Parents (round and angular. 



Hybrids (all round). 



Progeny obtained by sell- 

 fertilisation of hybrids 

 (three round to one angu- 

 lar). Of the three round 

 one is pure and the other 

 two hybrids. The angu- 

 lar is pure. These facts 

 are proved by the subse- 

 quent generations as in- 

 dicated. 



The numerical proportions i yy, 2 yg, i gg follow, of course, from 

 the assumption that there are equal numbers of <Jy's and c?'g's and 

 of $ y's and $ g's, each male gamete having therefore an equal chance, 

 presuming there is no disturbing element, of mating with a yellow or 

 a green female gamete. Owing to the dominance of the yellow, the 

 yy's and the yg's will all appear yellow, while the gg's will be green, 

 thus producing the constantly recurring Mendelian ratio of 3 : 1. 



If two pairs of characters are combined in the hybrid, say yellow 



and green cotyledons, and round and angular (wrinkled) shape of 



seed, then the germ-cells will be of the constitution yr, ya, gr, ga, 



and the possible combinations will be : 



cT yr 



^ YA 



' M-itK ' 



with 



S GA 



J ( 



YR 

 YA 

 GR 

 GA 



3 



