VARIATION AS THE BASIS FOR HEREDITARY DISTINCTIONS 



315 



evidence that each mutation is the result of a sudden change in some 

 previously existing gene and that only a single individual gene is con- 

 cerned. Since the gene that underwent the mutation occupied one definite 

 point (locus) in a chromosome, the mutant gene will now occupy that 



Forked 



Dichaete 



Stubble 



Miniature Scute crossveinless cut 



Fig. 21.2. Some of the variations which have arisen by mutation in Drosophila. {From 

 Sturtevant and Beadle, Introduction to Genetics, by per?nission W. B. Saunders Company.) 



point in all the chromosomes that are derived from the one in which the 

 mutation took place. 



We have no definite knowledge of what a gene may be and therefore 

 cannot, of course, know what kind of change it is that produces a muta- 



character, and it is still occasionally used in that sense, but today it is coming to be 

 restricted to gene or "point" mutations, as distinguished from variations associated 

 with chromosomal changes. 



