12 



Segregation 



[CH. 



DD X RR gives all DR 



Z^T? X 7^7? gives \DR 



DR X Z^T^ gives \DD : \DR 

 DR X DR gives iZ^Z^ : 2/^7? 



RR 



RR 



appearing as 3 dominants : i recessive. 



f; 



1^ 



^ 



F ^ 



•2 R 

 Batio 



IRR 

 IR 



D 

 R 



D 

 R 



X 



JicuUo ID 

 II 



IR 



R 

 R 



R 

 R 



cdlD 

 III 



Fig. 2. Diagrams showing numerical consequences of segregation. 



I. The mating DD x RR, and DR x DR. 11. The mating DR x i?i?. 



III. The mating DR x Z>Z). 



The way in which these ratios are produced may be 

 easily represented by means of a number of draught-men. 

 Pairs of draughts then represent zygotes ; single draughts 

 represent germ-cells. That there is a propriety in repre- 

 senting zygotic or somatic cells as double structures and 

 germ-cells as single structures will be evident to biologists ; 

 for we know that each somatic nucleus in plants and 

 animals is a double structure, containing twice the number 

 of chromosomes present in each mature germ-cell. Two 

 black draughts may then be taken to represent a pure black 

 individual, two white draughts a white individual. When 

 they are crossed together F, is represented by a black 



