8o 



GENETIC VARIATIONS 



are found to be unexpectedly frequent. In cases where the 

 same gene has mutated in different ways in different cases 

 (yielding what are known as multiple allelomorphs), the 

 inversions are found to be visibly of somewhat different 

 extent, or of different type, in the different cases. 



An example of these relations, as described by MuUer and 

 his associates, is found in the mutations of the so-called scute 

 gene. This is a gene that affects the bristles and their distri- 

 bution on the body, in Drosophila. From genetic evidence it 

 has long been known that this scute gene is at, or close to, the 

 tip of the X-chromosome. This would bring it close to the 

 first or second of the bands at the left or upper end of the 

 salivary X-chromosome, as seen in Figure 13. 



This gene scute has undergone many mutations — some 

 twenty to thirty — each giving a slightly different effect. Now 

 comes the essential point. In the stocks showing a mutation 

 of the scute gene, there is found to be a change in the thick 



Figure 1^. Diagrams of changes in the order of the bands in the 

 salivary X-chromosome of Drosophila, in connection with mutations 

 in the gene scute. A, B and C represent a portion of the extreme left 

 or upper tip of the X-chromosome. A. The normal arrangement of the 

 bands. B. Inversion of bands i and 2, in connection with one of the 

 mutations. C. Rearrangement of the bands in connection with another 

 mutation of the gene scute. 



