Gene Arrangement; Crossover Maps 147 



10.11. Describe the practical significance of the (act that linked genes are arranged 

 linearly. 



10.12. How can you determine the position of a centromere in a linkage group of 

 Neurospora? 



10.13. Under what conditions are all eight ascospores from a single sac detectable 



crossovers? 



10.14. In corn, virescent seedlings, glossy seedlings, and variable steriles are due to 

 three recessives — \\ gl, and va, respectively. G. W. Beadle test crossed a tri- 

 hybrid and obtained the following phenotypic results: 



235 normal 270 v gl va 

 40 va 48 v gl 



60 v 62 gl va 



7 gl 4 v va 



Construct a crossover map for these three genes, giving the map distances be- 

 tween loci. 



10.15. What effect do undetected multiple crossover strands have upon gene sequence 

 of marked loci? Observed map distance for marked loci? 



10.16. What evidence can you present that crossing over involves segments of homologs 

 rather than individual loci? 



10.17. Explain the following statement: The frequency of first division segregation of a 

 gene pair in Neurospora is inversely related to its distance from the centromere. 



10.18. How can you convert the percentage of asci showing second division segregation 

 into map distance from the centromere? 



10.19. In Neurospora, the mutant ag cannot synthesize the amino acid arginine, and the 

 mutant th fails to synthesize the vitamin thiamin. Since both substances are 

 needed for survival, explain how a stock pure for ag th is maintained and tested 

 for the presence of these mutants. 



10.20. A dihybrid for the mutants described in Question 10.19 produces asci having the 

 following spore orders: 



Spores 



Discuss the positions of these loci with respect to each other and their centro- 

 mere(s). How would you determine the genotypes present in these spores? 



10.21. Map mutant a relative to its centromere when Neurospora heterozygous for this 

 mutant produces asci having the following spore orders: 



Spores 



