GENE MUTATIONS AND EVOLUTION 8i 



second band at the tip of the X-chromosome (see Figure 14) ; 

 a change sometimes affecting other bands near by, also. In 

 Figure 14, A, B, C, are shown diagrams, based on Muller's 

 description, of the changes in the bands in certain well known 

 mutations of the scute genes. Figure 14, A, shows the usual 

 condition in the normal chromosome, in which there has 

 been no mutation of the scute gene. Figure 14, B, shows the 

 situation in one of the well known mutations of this gene, 

 causing a change in the bristles. The first and second zones 

 have interchanged places. That is, the chromosome has been 

 broken close to the tip, and there has been an inversion of 

 small extent. (Besides the mutation in the scute gene, con- 

 nected with band 2, there was also another mutation, a lethal 

 one, probably due to the change in the gene connected with 

 band i.) 



Another mutation of this scute gene was accompanied by 

 an additional mutation in a gene close to it, affecting the 

 facets of the eye. In this case there had been a break and 

 inversion affecting bands 2, 3 and 4; the condition is shown in 

 our Figure 14, C. A piece has been broken out and inverted. 

 At the upper end the break has gone through the middle of 

 the thick second band, leaving part of it in place. At the 

 lower end the break was between bands 4 and 5. So the order 

 of the bands now is: 



I — half of 2 — 4 — 3 — other half of 2 — 5 — 6 — etc. 



In another mutation of the scute gene a part of the thick 

 second zone was found to have been broken out and attached 

 in another region of the chromosome. 



These large salivary chromosomes are as a matter of fact the 

 two chromosomes of a pair united lengthwise, the corre- 

 sponding bands uniting throughout. Traces of the line 

 separating the two are observable, and in some cases the two 



