cytology: the study of the cell 139 



4 



1234567126 7126 71254367 



Fig. 3. Diagrams to illustrate one way in which chromosome structure can be 

 altered. Some of the genes of the chromosome diagramed are designated 1 to 7. 

 At the left, an unaltered chromosome. In the next diagram, the chromosome has 

 formed a loop. In the diagram at the right the chromosome has straightened out, 

 but some of its genes now lie in reverse order (an "inversion" has occurred). 



of segments, cytologists were able to demonstrate that corresponding seg- 

 ments of certain chromosomes had suffered the same alteration (fig. 3, 4). 



In recent years, the cytologist has been greatly helped in this work by the 

 discovery in certain organisms of giant chromosomes, whose structure can be 

 relatively easily analyzed. Thus, in the fruit fly, one of the most completely 

 analyzed organisms from the standpoint of genetics, the chromosomes in the 

 salivary glands and other organs of the larva become enormously swollen. 

 They show under the microscope a banded structure, with thinner or thicker 

 bands arranged in a constant pattern so that the cytologist can analyze this 

 pattern and learn to recognize each chromosome and chromosome part 

 (fig. 5). When a portion of a chromosome becomes inverted, or is exchanged 

 for a part of some other chromosome, the geneticist is able to show by his 

 analyses exactly what alteration has occurred, and the cytologist is able to 

 demonstrate exactly what parts of what chromosomes have been affected. 

 Thus it has been possible to relate each block of genes to a given chromo- 

 some, and even to place individual genes in the cytological chromosome. 

 There is a possibility that each band in the salivary-gland chromosome cor- 

 responds to a gene or a group of closely associated genes. Numerous genes 

 have actually been equated with specific bands in the chromosome. In 

 plants, no such giant chromosomes have been found, but there are cases 

 where a fair amount of correlation can be demonstrated. In maize, McClintock 

 and others have been able to relate many areas in the gene map, as deter- 

 mined genetically, with corresponding areas in the chromosomes, as seen 



Fig. 4. Another type of alteration in 

 chromosome structure ("transloca- 

 tion"). A chromosome whose ends are 

 designated 1 and 2 exchanges segments 

 with a non-corresponding chromosome 

 with ends 3 and 4. As a result new 

 chromosomes are formed, with ends 1 

 and 3, and 2 and 4. 



