MENDEL'S LAW OF HEREDITY 



as originally, L with W and S with D, or in a 

 new relationship, L with D and S with W. 

 Hence we expect the F individuals to form 

 four kinds of gametes all equally numerous, - 

 L W, S D, L D, and S W. By chance unions of 

 these in pairs nine kinds of combinations be- 

 come possible, and their chance frequencies will 

 be as shown in Fig. 31. Four of these com- 

 binations, including nine individuals, will show 

 the two dominant characters, short and dark; 

 two classes, including three individuals, will 

 show one dominant and one recessive charac- 

 ter, viz. dark and long; two more classes, in- 

 cluding three individuals, will show the other 

 dominant and the other recessive character, 

 viz. short and white; and lastly, one class, in- 

 cluding a single individual, will show the two 

 recessive characters, long and white. The four 

 apparent classes, or, as Johannsen calls them, 

 phenotypes, will accordingly be as 9:3:3:1. 

 This is called the normal Mendelian ratio for 

 a dihybrid cross, that is, a cross involving 

 two unit-character differences. 



One individual in each of these four classes 

 will, if mated with an individual like itself, 



45 



