Crossing Over and Chiasma j27 



QUESTIONS FOR DISCUSSION 



16.1. Does the Summary and Conclusions section contain one or more statements which 

 have not been made in essentially the same terms earlier in the Chapter? If your answer 

 is yes, give an example. 



16.2. In a species where both sexes undergo crossing over with equal frequency, what is 

 the percentage of crossing over between two loci if a mating between identical dihybrids 

 {Ab/aB) gives four equally viable classes of offspring, the smallest class comnrisine 

 1% of all offspring? ^ ^ 



16.3. How would you proceed to prove genetically that the last division in a spore sac of 

 Neurospora is a mitotic one? 



16.4. Could you determine, in the absence of crossing over, whether the alternatives for 

 two different traits were due to a single pair of genes or to two pairs of linked genes'^ 

 Explain. 



16.5. Draw an attached-X chromosome of Drosophila heterozygous both for y and for m. 

 Show the kinds of gametes which could be obtained following: 



a. No chiasma. 



b. One chiasma between the nonallelic genes. 



c. One chiasma not between the genes mentioned. 



16.6. Suppose a plant has a long pair of chromosomes, one member of which has a large 

 knob at one end of the chromosome while the other member has a small knob at the 

 opposite end of the chromosome. Suppose, moreover, there is also a shorter pair of 

 homologs, one member terminating with a large knob, while the other member termi- 

 nates at the other end with a small knob. 



With respect to these chromosomes, what combinations and configurations would 

 you expect to find readily in the gametes of this individual? 



16.7. What reasons can you present for believing that germ-line crossing over is based neither 

 upon premeiotic nor upon postmeiotic events? 



16.8. Translate into English the title of the article written by C. Stern. 



