Questions and Problems 389 



QUESTIONS AND PROBLEMS 



1. In an experiment on maize, Jones started with a variety of Learn- 

 ing dent corn yielding 88.0 bushels per acre. After ten generations of 

 selfing, one line had a yield of 32.8 bushels and another of 31.8. A third 

 line reached 59.3 bushels after two generations. Two plants were selfed 

 from this point on; after eight more generations, one yielded 32.7 bushels 

 per acre and the other 19.2. Explain. 



2. If maize is inbred until a number of homozygous hues have become 

 established, would all single crosses between the various lines show the 

 same degree of heterosis? Explain. 



3. In cotton, selection had developed excellent late-maturing types. 

 When the boll weevil arrived, these late-maturing varieties could not be 

 grown in spite of their superiority in other ways. Some years later the 

 cotton wilt arrived and spread over large areas. How would such events 

 affect a cotton-breeding program? 



4. Would selection be easier in a self-fertilized crop such as wheat or 

 a cross-fertilized one such as maize? Explain. 



5. Assume that six genes. A, B, C, D, E, and F, add 4 cm each to 

 the height of a 50-cm plant, while their alleles add nothing. These 

 genes are dominajit. A cross is made between two plants whose geno- 

 types are AA bb CC dd ee and aa BB cc DD EE. What is the height of 

 each of these plants? Does the Fi show heterosis. Explain. 



6. Assume that a plant has twelve dominant genes each of which adds 

 2 cm to the height of a plant 50 cm tall. These genes are on two chromo- 

 somes. The following cross is made: 



AbCdEf GhljKl aBcDeF gHiJkL 

 AbCdEf GhljKl aBcDeFgHiJkL 



What would be the height of the Fi ? 



7. Assume the cross in problem 6. How many plants would be neces- 

 sary in the F2 to be sure to get a homozygote as large as the Fi plants 

 if there was no crossing over. Would the result be different if there was 

 crossing over? 



8. Assume the cross in problem 5. Starting with the Fi, you wish to 

 obtain two pure-breeding types, one of which is 74 cm tall and the other 

 50 cm. How would you go about establishing such lines? Would either 

 line be easier to establish than the other? Explain. 



9. In establishing the hnes in question 8, would it speed up the 

 process if the selections for the tall line were grown in good, rich soil, 

 whereas those for the short line were grown in poor, barren soil? 

 Explain. 



