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AN INTRODUCTION TO MODERN GENETICS 



which come near to falling into this category; one may mention Bar, 

 which was only recently discovered to be a duplication by very careful 

 examination of salivary gland chromosomes. Very minute inversions 

 are likely to be even more difficult to identify. If the affected segment 

 were below the limit of resolving power of the microscope, there is at 



Fig. 152. Small Rearrangements in the left end of the X chromosome, as seen 

 in salivary gland chromosomes. The "factors" scute 19, yellow 3P, scute 8 and 

 scute A are cases in which minute sections of rings 2 and U have been deleted or 

 translocated to other regions. The evidence indicates that ring 2 must have 

 a compound structure, as in the diagram, and it is actually possible, by ultra-violet 

 photography, to demonstrate this. Lethal 71-scute jl is a minute inversion of 

 rings 1 and 2, while achaete 3 is an inversion stretching from within ring 2 to 

 |ust above ring 5. 



(From Muller and Prokofieva, and Muller, Prokofieva, and Raffel.) 



present no known way in which a position eflfect could be distinguished 

 from a gene mutation. Possibly many of the so-called gene mutations 

 will turn out to be minute rearrangements. 



Some authors have concluded from this that all mutations are really 

 rearrangements of the chromatin, and that we must give up the idea of 

 the gene and substitute for it the concept of the chromosome as a whole. ^ 

 This pessimism appears extremely premature. The position effect is a 

 comparatively rare one, known only in Drosophila, where it only 

 1 Cf. Goldschmidt 1938. 



