Cytological Methods 189 



As these methods are of interest to advanced rather than be- 

 ginning students, they are not discussed here. 



Cytological Methods 



A linkage group shows the genes linked with one another, the 

 order in which they are arranged and the distance they are apart 

 from one another as determined by genetic means. Genetic 

 methods alone, however, are not sufficient to show exactly which 

 chromosome as observed through the microscope corresponds to 

 a given linkage group, although sometimes it may be inferred 

 from the size of the linkage groups and the size of the chromo- 

 somes. Combined genetic and cytological studies, utilizing either 

 spontaneous chromosomal abnormalities or similar aberrations 

 induced by radiation, have in some cases supplied this missing 

 information. 



In Drosophila melanog aster there are four linkage groups. 

 Two are very long and almost the same size, one is extremely 

 short and includes only a few genes, and the fourth is inter- 

 mediate in length. A cytological examination of this species 

 shows two pairs of very long chromosomes that are almost the 

 same size, a pair of very short ones, and the intermediate-sized 

 chromosome, which is paired in the female and single in the 

 male, and is the X chromosome. In the male, a Y chromosome 

 is also observed cytologically. It is reasonable to correlate the 

 very small linkage group with the very short fourth chromosome. 

 The fact that the genes of the intermediate-sized linkage group 

 are sex-linked establishes definitely the connection between this 

 linkage group and the intermediate-sized chromosome. It is im- 

 possible, however, to state which of the large linkage groups 

 corresponds to each of the pairs of large chromosomes merely 

 by looking at them. By observing a series of translocations, 

 however, Dobzhansky was able to resolve this' difficulty. 



A translocation is a chromosomal aberration in which a piece 

 of one chromosome becomes broken off in some manner and 

 becomes attached to another chromosome, frequently at the end. 

 If the translocated piece includes genes, these genes no longer 

 show linkage relationships with the genes with which they were 

 previously linked and now show linkage and crossing over with 

 the genes of the chromosome to which they had become at- 

 tached. In other words, translocation changes the structure of 



