98 AN INTRODUCTION TO MODERN GENETICS 



attachment constriction and from the free end of the chromosome 

 (cf p. 125). 



The maps based on mutation frequency are not affected by regional 

 differences in crossing-over and agree very much better with maps 

 based on the metaphase lengths of translocations, etc. 



5. Salivary Gland Chromosomes 



The metaphase chromosomes of Drosophila possess few external 

 marks which can serve as fixed points for mapping. More detailed 

 maps can be made of organisms in which the chromosomes show mor- 

 phological differentiation along their length, for instance by possessing 

 visible chromomeres. Further, at metaphase the gene thread or chro- 

 monema is coiled in a spiral and the length of chromosome occupied by 

 a section of chromosomes will depend on the tightness of the spiral, 

 while in the prophase stages where chromomeres are visible the spiral 

 is practically uncoiled and this source of error does not occur. Until 

 recently no organism was known in which both detailed genetical 

 analysis and well defined chromomere structures were available. But in 

 1933 attention was called to the chromosomes in the larval salivary 

 glands of Diptera^ and particularly of Drosophila,^ which have a very 

 pecuHar structure in which the linear differentiation is clearly visible. 



In the very large nuclei of the saUvary gland the chromosomes are as 

 though in permanent meiotic prophase; they are paired and are thus 

 present in the haploid number. Each chromosome, after the pairing has 

 taken place, seems to have divided many times, so that the paired 

 chromosomes consist of a series of some hundreds of threads lying 

 parallel, forming the walls of a hollow cylinder.^ The threads bear 

 chromomeres, which show as rows of dots around the cylinder, or may 

 apparently fuse into discs. The heterochromatic (inert) parts of the 

 chromosomes may in some species, of which Drosophila melanogaster 

 is one, fuse into a mass (the chromocentre) to which the euchromatic 

 parts are attached. The length of the chromosomes may be as much 

 as 100-150 times that of mitotic metaphase chromosomes; one can 

 probably regard the parallel threads of which they consist as the 

 completely unwound chromonemata which make up the short coil of 

 the metaphase chromosomes. 



The complicated chromomere structure and large size of the salivary 



1 Heitz and Baur 1933. ^ Painter 1934, 1935. 



' Some authors consider that the cylinder is solid or has a honeycomb 



structure, cf . Metz 1935, Rev. of structure and origin,Cooper 1938, Painter 1939- 



