Appendix E 429 



EXERCISE 13 



Practice in the Cross-pollination of Apples, Pears, Peaches, 



Plums, etc. 



To be carried on in the spring, when the trees are in bloom. 

 For general methods of procedure, see Exercise 10. 



EXERCISE 14 



Purpose. To teach the Laws of Probability ; dominance 

 and recessiveness ; segregation and recombination; presence 

 and absence hypothesis; inhibitory factors; complementary 

 factors ; inversed ratios, etc. 



Materials. Coins, wrinkled and smooth peas, both yellow 

 and green hi equal numbers for two character pairs ; yellow and 

 white kernels of both dent and flint corn; a pack of playing 

 cards ; and chemicals. 



Program. The instructor should take special care to make 

 clear the significance of each step in the exercise and their con- 

 crete application to problems of plant-breeding and genetics. 



1. The Law of Probability is taught by tossing coins. Each 

 student should toss one coin for 2 or 3 minutes and record the 

 number of tunes it falls head, and the number of times tail. 

 Then the total for the whole class is summed up. It will be 

 found that the latter count, including more tosses, approaches 

 the theoretical ratio much more nearly. This should be ex- 

 plained by the instructor. 



2. Then in the same way two corns may be tossed by each 

 student. He now records heads ; heads and tails ; tails. 



The application of this law in the formation of gametes should 

 be made clear by the instructor. 



3. Now the material may be changed by way of illustration. 

 Peas or corn comprising two allelomorphs may be used for this 

 exercise. They are mixed together in equal numbers in a bag 



