Cytological Methods 191 



a smaller frequency there, the linkage map for that region will 

 appear too short. 



The general method in use in attacking this problem is this. 



(1) By subjecting organisms to X-rays, pieces of the chromo- 

 somes are broken off and translocated to other chromosomes. 



(2) Genetic linkage studies are then undertaken to determine 

 at what place on the genetic map each break occurred. (3) The 

 chromosomes of each fly with a translocation are then examined 

 cytologically and the place of the break determined by noting 

 the nearness of the break to the end, to the centromere, and to 

 various secondary constrictions that may be present. (4) The 

 proportionate length of the broken piece is determined by 

 measurement. (5) The proportionate size of each translocation 

 is compared with the break in the linkage map of that fly. 

 (6) By these comparisons the approximate position on the chro- 

 mosome is determined for a number of genes. 



When a large number of translocations are studied the actual 

 spatial relationship of the genes of the linkage group can be 

 determined reasonably accurately. These translocations have 

 been studied for the metaphase chromosomes, and the cytological 

 map constructed by this means is called a "metaphase chromo- 

 some map." The student must not get the impression that the 

 cytologist can direct the X-rays so as to break a chromosome 

 at any desired point. He frequently w^ishes he could, but, so 

 far, it has been impossible. The breaks occur by chance and 

 the cytologist must take them as they come. If a sufficient 

 number of flies is treated, however, the chance is good of ob- 

 taining a number of breaks at different places in the same 

 chromosome. 



A comparison of a genetic linkage map and the corresponding 

 metaphase chromosome map of the X chromosome, chromosome 

 II, and chromosome III of Drosophila melanogaster (Fig. 56) 

 shows that in each case the order of the genes is the same but 

 that the genetic distances and cytological distances do not always 

 agree. In the two large chromosomes, genes near the centro- 

 mere are much farther apart on the metaphase chromosome than 

 w^ould be supposed from linkage studies alone. For example, 

 Dobzhansky has pointed out that on the second chromosome 

 the distances between the genes It (light eye), rl (rolled wings), 

 and tk (thick legs) amount to less than one one-hundredth of the 



