Cytological Methods 193 



same in genetic and cytological maps. For example, a large 

 number of genes appear to occupy a small part of one end of 

 the linkage map but to cover a relatively much greater part 

 of the metaphase map. In general, it seems that the genes on 

 the Drosophila chromosomes are much more evenly spaced on 

 the metaphase map than on the map constructed from crossover 

 data. 



Essentially the same techniques which are used in the con- 

 struction of metaphase chromosome maps are also used to com- 

 pare the order and distance of genes on the linkage maps of 

 Drosophila melanogaster with the order and position of the 

 bands on the salivary gland chromosomes. It was pointed out 

 in Chapter 5 that these giant chromosomes are characterized by 

 the presence of a number of bands or discs of different size and 

 staining capacity which are separated by regions of nonstaining 

 material. Some bands are thick and very deeply stained with 

 aceto-carmine ; others are thin and very lightly stained. P. N. 

 Bridges has counted 3795 such bands (counting as one certain 

 bands that appear double when the chromosomes are greatly 

 stretched). 



The important problem is whether these bands indicate the 

 regions in which the genes are located. The method of study 

 involves considerable labor. The first problem was to study the 

 morphological features of the normal salivary gland chromo- 

 somes and determine the position and order of each band and 

 its relation to gross permanent features of the chromosomes, 

 such as swollen portions and thinner regions. Once this was 

 known, the next steps were to produce translocations, inversions, 

 and deletions. In all cases, the material had to be known 

 genetically, so that it could be determined from phenotypie 

 studies where the breaks in the linkage groups occurred. Cyto- 

 logical observation was then made to determine which bands 

 or groups of bands had changed position. 



If a piece is broken from the interior region of one chromo- 

 some (an intercalary deletion), the remaining parts pair with 

 the corresponding parts of the homologous chromosome band 

 for band. The part of the normal chromosome which corre- 

 sponds to the part that is missing from the deleted chromo- 

 some bulges out to one side and takes no part in pairing (Fig. 

 57). Cytological observations show which bands are missing 



