Appendix E . 433 



liminary experiments to see whether or not the strengths of the 

 solutions are correct. They may have to be varied slightly. 

 The contents of each test tube representing a gamete (labeled in 

 the accompanying figures) are given below. In order to secure 

 the simple 3 : 1 or 1 : 3 ratio in F 2 , eight test tubes representing 

 the gametes of FI are necessary in each case. It is of course 

 impossible to represent the phenomenon 'of segregation in FI 

 by using the test tube labeled FI. The instructor will have to 

 explain that after segregation the gametes are exactly the same 

 in nature as those of the original parents of the cross, and that 

 the hybrid FI now forms gametes similar to those of both parents, 

 in equal numbers. 



(a) Demonstration of 'Allelomorphism and of Complete 

 Dominance (Fig. 110). 



D contains 10 cc. 10 % HCl + 2 cc. litmus solution. 

 R contains 10 cc. 10% NH^OH + 2 cc. litmus solution. 



The dominance of blue over red can be shown by substituting 

 5% HCl for the 10 %. 



(6) Demonstration of the Presence and Absence Hypothesis 

 and of Intermediacy (Fig. Ilia). 



A contains 10 cc. 10 % NH^OH + 2 drops Phenolphthalein. 

 a contains 10 cc. 5 % HCl. 



(c) Demonstration of Complementary Factors (Fig. lllb). 

 A contains 10 cc. 10% NH 4 OH. 

 B contains 10 cc. H - 2 + 2 drops phenolphthalein. 



Dominance of a character has usually been taken to be indica- 

 tive of the presence of a positive factor determining that char- 

 acter. But in some cases the absence of a factor, e.g. cases of 

 awnlessness in wheat, or hornlessness in cattle, seems to be 

 dominant over its presence. To say that the absence of a thing, 

 in other words a purely negative condition, is dominant over its 



