GENE MUTATIONS AND EVOLUTION 79 



visible certain relations that a few months ago one would 

 have expected to remain forever hidden. 



These great chromosomes, with their visible structure, 

 provide a means of determining definitely whether certain 

 chromosomes have been broken, with the production of 

 deletions, inversions, translocations, and the like, for these 

 things are clearly seen in the changed order of the bands. 

 They make it possible to determine whether known gene 

 mutations are accompanied by a break in the chromosome at 

 or near the point where the mutation occurs. 



This has already been determined for certain well known 

 mutations. We have before seen that when a break is known 

 to have occurred in a chromosome, this break is usually 

 accompanied by a gene mutation. But many gene mutations 

 have long been known with no corresponding knowledge of 

 a break in the chromosome. Examination of the salivary 

 chromosomes in stocks carrying such known mutations has 

 shown for certain cases that the mutation is indeed accom- 

 panied by a break at or near the point of mutation; this break 

 is presumably what gave rise to the mutation. 



This has been demonstrated by H. }. MuUer and his asso- 

 ciates.^" They have shown that a number of long-known 

 mutations involve an inversion in the order of the bands of 

 the corresponding salivary chromosome; including the band 

 known to carry the material of the mutated gene. Such an 

 inversion is known to be the result of chromosome breakage, 

 as described on earlier pages. The inversions are found to be 

 usually of small extent, often involving but two or three genes 

 or chromomeres. It is of course important to remember that 

 at the time the break and inversion occur, the chromosome is 

 in the elongated, thread-like condition indicated in the dia- 

 grams of Figures 11 and 12; at such times inversions of small 

 extent are readily possible. Such inversions of minute extent 



