12 



Segregation 



[CH. 



DD X RR gives all DR 



DR X RR gives iZ^i? : ii?7? 



DR X Z>Z^ gives iZ^Z? : \DR 



DR X Z>7? gives iDD : 2DR : iRR 



appearing as 3 dominants : i recessive. 



]^: 



]R 



"2 r: 



BcuUo 



: c 



ID 

 DR 



IRR 

 IR 



2DR 



IDD 



3D 



Rt 



II III 



Fig. 2. Diagrams showing numerical consequences of segregation. 



I. The mating DD x RR, and DR x DR. II. The mating DR x RR. 

 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 



