194 



CHAPTER 22 



FIGURE 22-6. Drosophila melanogaster males: normal {A), bithorax (B), postbitho- 

 rax (C), and bithorax postbithorax (D). (Courtesy of E. B. Lewis; reprinted by per- 

 mission of McGraw-Hill Book Co., Inc., from Study Guide and Workbook for Genetics 

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



products of their alleles in the homologous 

 chromosome, for this homolog is usually 

 located a considerable distance away. Con- 

 sider also cases of position effect brought 

 about by structural changes. The same 

 reasoning could explain position effects fol- 

 lowing shifts in the relative positions of 

 heterochromatin and euchromatin via break- 

 age. In fact, position effects from structural 

 changes would be expected to be particu- 

 larly common in species whose chromosomes 

 or chromosome parts are not located at ran- 

 dom in the nucleus, but take on special posi- 

 tions relative to each other. The facts that 

 during nuclear division Drosophila chromo- 

 somes show somatic synapsis, and that 



somatic synapsis is found in the interphase 

 nuclei of salivary gland and other cells, sug- 

 gest that at the time of gene action different 

 chromosomes and their parts are arranged so 

 that the products of gene action may be 

 formed or used in particular sequences. 

 Intra- and interchromosomal rearrange- 

 ments which change these sequences might 

 be expected to produce position effects. 

 The fact that Oenothera chromosomes form 

 a circle of 14 during meiosis (Chapter 20) 

 demonstrates a very orderly arrangement of 

 chromosomal parts involving heterozygosity 

 for reciprocal translocations. Here also, a 

 new arrangement of chromosomal parts 

 might be expected to disturb functional se- 



