136 CHAPTER 17 



REFERENCES 



Sturtevant, A. H., "The Linear Arrangement of Six Sex-Linked Factors in Drosophila, as 

 Shown by Their Mode of Association," J. Exp. Zool., 14 : 43-59, 1913. Reprinted in 

 Classic Papers in Genetics, J. A. Peters (Ed.), Englewood ClifTs, N.J., Prentice-Hall, 

 1959, pp. 67-78. 



See Supplement IL 



QUESTIONS FOR DISCUSSION 



17.1. Does the linear arrangement of the genes offer any evidence for or against the view 

 that the chromosomes are carriers of genes? Explain. 



17.2. How many gene pairs must be heterozygous to detect (1) a single crossover; (2) a 

 double crossover in Drosophila! in Neurosporal 



17.3. Suppose a pair of homologs in Neurospora had the genotypes AB/a b. Draw an 

 ascus, containing 8 spores, derived from a cell that had: 



a. No chiasma between these homologs. 



b. One chiasma between the centromere and the gene pair closest to it. 



c. One chiasma between the two pairs of genes. 



d. One two-strand double chiasmata between the two pairs of genes. 



17.4. What advantages has Neurospora over Drosophila as material for genetic studies? 



17.5. Under what conditions would segregation of a pair of alleles occur during the first 

 meiotic division? the second meiotic division? 



17.6. What indications would you have that dilTerential viability was having a role in 

 modifying the crossover distances obtained experimentally? 



17.7. For how many linked gene pairs must an individual of Drosophila and of Neurospora 

 be hybrid, in order to determine from the crossovers it produces whether these genes 

 are arranged in a linear sequence? Explain. 



17.8. A test cross proves that one of the parents produced gametes of the following geno- 

 types: 42.4% PZ, 6.9% Pz, 7.0% pZ, and 43.7% pz. List all the genetic conclu- 

 sions you can reach from these data. 



17.9. The trihybrid Aa Bb Cc is backcrossed to the triple recessive, aa bb cc. The following 

 phenotypic results were obtained: abc 64, abC 2, aBc 11, aBC 18, AbC 14, Abe 17, 

 ABc 3, ABC 71. 



a. Which of these loci are linked? Why? 



b. Rewrite the genotypes of both parents. 



c. Determine the observed map distances between all the different pairs of linked 

 genes 



17.10. Describe one practical use of the fact that linked genes are arranged linearly. 



17.11. How can you determine the position of a centromere in a linkage group of Neuro- 

 spora? 



17.12. Discuss the statement on p. 124 that "never are all eight spores, from a single sac, 

 crossovers." 



