190 Chromosome Maps 



the chromosomes and also the composition of the linkage groups. 

 Several of Dobzhansky's translocations involved segments of 

 the third chromosome, which became attached to the X chromo- 

 some or to the very short chromosome IV. Chromosome III is 

 the longer of the two long chromosomes. By observing which 

 genes had changed their linkage groups, Dobzhansky was able 

 to determine which linkage group corresponded to the longest 

 pair of chromosomes. Similar translocations of pieces of the 

 second chromosome established the correspondence between the 

 shorter of the pair of long chromosomes and the genes of the 

 second linkage group. 



Translocations have also been used to locate individual genes 

 in definite regions of their chromosome. An interesting example 

 is Stern's determination of the location of genes for male fer- 

 tility in Drosophila melanog aster. Males with both an X chro- 

 mosome and a Y chromosome that has lost its short arm are 

 sterile. Other males which have lost the long arm of their Y 

 chromosome are also sterile. When both arms of the Y chromo- 

 some are present, the male is fertile. It follows that the two 

 arms of the Y chromosome contain genes for fertility and that 

 genes of both arms must be present if the fly is to be fertile. 

 These fragments of the Y chromosome arose spontaneously. 



By the use of a large series of translocations the position of a 

 number of genes on the chromosomes has been shown. Although 

 crossover data give the sequence of the genes and the distance 

 between them, these distances are calculated purely statistically 

 and must necessarily be based on the assumption that crossing 

 over occurs with equal ease in all parts of the chromosome. 

 AVhether these distances and the genetic maps based upon them 

 correspond to the true spatial relationships of the genes on the 

 chromosomes depends entirely upon the correctness of this as- 

 sumption. If the frequency of crossing over per unit of chromo- 

 somal length is different in different parts of a chromosome, the 

 linkage map wdll not give a picture of the exact distribution 

 of the genes in space on that chromosome. If the genes cross 

 over more frequently in one particular region of the chromo- 

 some, the linkage map for that region will be too long; and if 

 in another part of the chromosome inert material is present in 

 which crossing over does not occur so that crossovers occur with 



