62 HEREDITY [ch. 



of white ; the whites and one in every three of the 

 purples so produced are 'extracted' homozygotes, 

 being pure for whiteness or purpleness respectively, 

 and therefore breeding- true, while the remaining 

 purples are heterozygous and when ' selfed ' will give 

 both colours among their ofispring. 



If now a tall purple-flowered pea is crossed with 

 a short white-flowered, the heterozygous offspring 

 will be tall with purple flowers, for both these 

 characters are dominant. In the production of 

 their gametes (pollen-cells and egg-cells) segregation 

 will take place between tallness and shortness, and 

 between purpleness and whiteness, but as these pairs 

 of characters are totally independent of one another 

 they may be associated in any combination as long as 

 both members of a pair do not occur in the same 

 gamete. Gametes will thus be produced of four 

 kinds ; if P represents purple, p its absence (white) ; 

 T tallness and t its absence (shortness), the gametes 

 produced by an individual heterozygous in both 

 characters will be PT,pT, Pt, pt, with equal numbers 

 of each. Since these will meet one another at random 

 in fertilisation, the F^ generation will consist of 

 individuals (zygotes) made up of all possible com- 

 binations of these four types of gametes, viz. in the 

 proportion of 4.PpTt, 2PpTT, 2PPTt, \PPTT; 

 2Pptt, \PPtt ; 2p27Tt, \ppTT ; \pptt 



Since purple is dominant over white and tall over 



