A Simple Chemical Device 



151 



taining the other red solution is taken from the "male" rack and 

 its contents poured into one of the blue solutions on the "fe- 

 male" rack. When all the combinations have been made the 

 four test-tubes on the' * female " rack represent the second genera- 

 tion hybrids and show 3 red to 1 blue. That one of the three 

 red individuals of this second generation was a positive homo- 

 zygote and the other two were heterozygotes, should also have 

 been carefully noted by the class. 



(B) To demonstrate the dominance of absence over presence: 

 Starting again with the two flasks containing an alkaline solu- 

 tion of litmus, determine how much acid must be added just 

 to make the solutions safely acid; then take half of this quantity 

 to represent the acid-producing unit coming from each of the 

 parents. When this half portion of acid has been added the 

 solution should remain blue, but on adding the second half 

 portion the color will change to red, so that now you are again 

 prepared to make a cross between a blue and a red individual 

 The rest of the experiment is conducted exactly as in (A), but 

 the first generation will show four blues, and the second generation 

 will show 3 blue to 1 red, that is, the absence of the added acid 

 unit is dominant over its presence. 



(C) To show the equivalence of reciprocal crosses, regardless 

 oj the fact that egg and sperm differ much in size- Take the situa- 

 tion described under (A) after preparation has been made for 

 the demonstration of the second generation. Fill up the test- 

 tubes on the female rack with water until they are half or 

 two-thirds full, leaving those on the male rack one-fourth 

 filled with the more concentrated fluids. As this is done it 

 should be explained that just as the water is neither acid nor 

 alkaline and does not modify the reaction which takes place when 

 the two fluids are brought together, so the food-materials whose 

 accumulation makes the egg larger than the sperm are neutral 

 substances which do not affect the behavior of the hereditary 

 qualities. If desired, this difference between the egg and 

 sperm may be represented throughout all of these experiments, 

 by adding water to the test-tubes on the female rack in all 

 cases, but as the aim should be to keep the demonstration as 

 simple as possible, it would seem best usually to follow the plan 



