192 



Chromosome Maps 



genetic map but actually cover one-fourth of the metaphase 

 chromosome. Similarly for chromosome III, st (scarlet eye) 

 and cu (curled wings) occupy about one-eighteenth of the genetic 

 map but one-fifth of the metaphase chromosome. Other dis- 

 crepancies appear also in other parts of these chromosomes. For 

 these two chromosomes, genes near the centromere are relatively 

 farther apart on the metaphase map than on the genetic map; 



m G 



Fig. 56. A comparison of genetic and cytological maps of the X, second, 

 and third chromosomes of Drosophila melanogaster. G, genetic map; S, 

 salivary gland map; O, mitotic map. Centromere is marked s.f. Corre- 

 sponding loci on the three types of maps are connected by transverse lines. 

 (Redrawn from Mather in Biological Reviews.) 



the genes in the middle region of either arm of both chromo- 

 somes are relatively farther apart on the genetic map; those 

 near the ends of the second chromosome are relatively farther 

 apart on the metaphase map. 



Discrepancies also appear when the X chromosome is studied 

 and are even more marked, for only one gene, bobbed, is present 

 in the third of the chromosome nearest the centromere. This 

 part of the chromosome is the "inert" region and, although it 

 appears to be a normal part of a chromosome when the meta- 

 phase chromosome is examined under the microscope, it forms 

 part of the chromocenter of the salivary gland chromosomes and 

 therefore must be different from the rest of the chromosome (see 

 Chapter 5). The remaining two-thirds of the chromosome con- 

 tains all the known genes except bobbed, but even in this part 

 of the chromosome the distances between the genes are not the 



