S3 



78 GENETIC VARIATIONS 



jj£^ But recently Painter^ has discovered that the bands 



m, and zones of the salivary chromosomes mark the po- 

 )Si sition of definite genes known from their effects in 

 ^ inheritance. In general, the banded structure cor- 

 responds to the known arrangement of genes and of 

 mutations. In the normal flies, not mutated, the bands 

 ^ of any chromosome show a typical order and typical 

 Q diversities in size and texture, as shown in Figure 13. 

 c In the different chromosomes of any species the 

 ^ banded structure differs; so that it is possible to recog- 

 Q nize the particular chromosomes by this structure. 

 Study of the method of inheritance of the charac- 

 teristics depending on known genes shows that in 

 certain particular chromosomes a certain series of 

 genes are reversed in their order, as compared with 





t;rr^ 



S the usual arrangement (as in Figure 11). When this 

 tiJ particular chromosome is studied as found in the 

 ^. salivary glands, it is discovered that in a certain region 

 the order of the bands is correspondingly reversed. It 

 thus becomes possible to know that the reversed genes 

 are connected with the reversed bands. Again, in 

 some cases a few genes are known, from genetic evi- 

 dence, to have been lost from a certain chromosome; 

 ^ in such cases this chromosome, as found in the sal- 

 ivary gland, is found to have lost certain bands. By 

 these and similar methods it has become possible to 

 determine with certainty that the materials of par- 

 ticular genes are located in certain particular bands, 

 t=t' having a definite position and structure. The great 

 chromosomes of the salivary glands make clearly 



*-■■-■, 



tv.J 



m 



:-.-\ 



r — 1 



^ 



,t=3. 



^ Figure /J. The enlarged X-chromosome of Drosophila mel- 



anogaster as found in the saHvary gland, after Painter, 1934. 



