Mechanism of Mendelian Heredity 239 



characters round and yellow. When these were inbred 

 the Fa generation produced four types of seed in the 

 ratio 9 : 3 : 3 : I, namely: 



(i) yellow round (315 seeds) 



(2) yellow wrinkled (loi seeds) 



(3) green round (108 seeds) 



(4) green wrinkled (32 seeds) 



The explanation according to Mendel's theory is as 

 follows: Since the segregation of each pair of char- 

 acters occurs independently, there must be 3 yellow to I 

 green and also 3 round to i wrinkled in the F^ genera- 

 tion. The yellow will, therefore, be round and wrinkled 

 in the ratio of 3 : i, which will give 9 yellow round to 3 

 yellow wrinkled. The green will also be round and 

 wrinkled in the ratio of 3 : i , which will give 3 green round 

 to I green wrinkled, which is the ratio of 9 : 3: 3: i 

 found by Mendel. 



On the basis of the chromosome theory the following 

 explanation could be given of this numerical relation. 

 The peas with yellow round seeds have sex cells with a 

 factor for both yellow and for round in two different 

 chromosomes; these two different chromosomes we will 

 designate with Y and R. The peas with green and 

 wrinkled seeds will have in their sex cells factors for 

 these characters in two homologous chromosomes g 

 and w, where g is the homologue of Y and w of R. 

 The cells of the hybrids of the Fi generation will have 



