194 



Chromosome Maps 



Deleted 



from the deleted chromosome, and genetic linkage studies indi- 

 cate which genes are missing. The assumption naturally is made 

 that the missing genes are located in the region of the missing 

 bands. Patterson has shown that short deletions can be pro- 

 duced with considerable frequency by subjecting flies to radi- 

 ation. Short deletions are naturally better for this type of 

 work than long ones for they restrict the number of genes and 



bands involved. By using a large 

 number of such deletions, Painter, 

 Mackensen, and others have succeeded 

 in locating the regions of a number 

 of genes. In addition to series of 

 overlapping deletions, translocations 

 and inversions are also used and yield 

 the same types of evidence. By these 

 various related methods, salivary gland 

 chromosome maps have been made for 

 the X chromosome and for both arras 

 of chromosome II and chromosome 

 III. The most recent maps have been 

 published by C. B. and P. N. Bridges 

 (Fig. 58). 



The translocation technique has re- 

 cently given us some interesting infor- 

 mation on the fourth chromosome of 

 Drosophila melanogaster. This chro- 

 mosome had generally been regarded 

 as a very tiny dot-like chromosome with the centromere at one 

 end, although the possibility of a strictly terminal centromere 

 has recently been very strongly questioned. Griffen and Stone 

 succeeded in breaking off a small piece of the X chromosome 

 just to the right of the five sharp bands which mark the ichite 

 locus; this piece then became translocated to the fourth chromo- 

 some. Metaphase studies of a fly heterozygous for this translo- 

 cation showed one normal chromosome IV and one that was 

 two-lobed, indicating that the translocated piece had apparently 

 become attached to the centromere end (Fig. 59a). An examina- 

 tion of the salivary gland chromosomes of such flies showed the 

 translocated piece up to and including the region of white, but 

 also revealed a region between white and the centromere of chro- 



FiG. 57. Pairing in a small 

 section of a salivary gland 

 chromosome of Drosophila 

 melanogaster in which one 

 chromosome has a deleted 

 segment. The correspond- 

 ing segment of the homol- 

 ogous chromosome forms a 

 loop. (Redrawn from Painter 

 in the Journal of Heredity.) 



