4S0 



CHAPTER 38 



figure 38-6. Drosophila melanogaster males: normal (A), bithorax (B), post- 

 bithorax (C). and bithorax postbithorax (D). (Courtesy of E. B. Lewis; reprinted 

 by permission of McGraw-Hill Book Co., Inc., from Study Guide and Workbook 

 for Genetics by I. H. Herskowitz. Copyright, 1960.) 



The normal wild-type fly (Figure 38-6A) 

 has small club-shaped balancers (halteres) 

 located on the posterior part of the thorax. 

 One of the pseudoalleles, bithorax (bx), 

 converts the haltere into a large wing-like 

 structure (Figure 38-6B); another, called 

 postbithorax (pbx). appears to do much 

 the same thing (Figure 38-6C). But close 

 examination reveals that these two recessive 

 pseudoalleles really have different func- 

 tions. Bithorax converts the front portion 

 and postbithorax the hind portion of the 

 haltere into a wing-like structure. Flies ho- 

 mozygous for both mutants demonstrate 



these changes in a fully developed second 

 pair of wings (Figure 38-6D). 



What are the cis-trans effects for bx and 

 pbx? The cis form (H — \- bx pbx) has 

 normal balancers, whereas the trans form 

 (bx -\-/+ pbx) shows a slight postbithorax 

 effect, providing another example of cis- 

 trans position effect and demonstrating the 

 nonallelism of these genes. The map dis- 

 tance between these loci is 0.02. 



These examples of pseudoallelism appar- 

 ently involve separate but closely linked 

 functional genes; they do not seem to in- 

 volve intragenic recombination of the type 



