Regulation of Gene Action — Position Effect in Drosophila 483 



38.6. What genotypic steps are required to prove that a region shows a cis-trans 

 position effect? Explain. 



38.7. Can one of the steps in question 38.6 be a lack of all or part of the region 

 under investigation? Why? 



38.8. What crosses would you make to test whether two recessive mutants in Dro- 

 sophila. apparently alleles of the X-linked gene v+ (normal allele of vermilion 

 eye color), are pseudoalleles? 



38.9. Using appropriate genetic markers, draw a tetrad configuration which would 

 permit you to identify strands which have undergone intrachromosomal ex- 

 change in the Bar region of the X chromosome of Drosophila melanogaster. 



38.10. Using the operon concept, explain how a paracentric inversion in the X chro- 

 mosome of Drosophila might result in a mottled eye color phenotype. 



38.11. Suggest a molecular explanation for the cis-trans position effect observed in the 

 white region of the X chromosome in Drosophila. 



38.12. Can position effect occur in haploids? Why? 



