44 GENETIC VARIATIONS 



evolution: whether the observed changes in the genes do 

 indeed provide the fundamental materials for progressive 

 evolution. 



The changes knovv^n as gene mutations have been found to 

 occur in genes that affect all parts and functions of the organ- 

 ism. Indeed, in the organism in which thecourseof heredity is 

 best known — the fruit fly — practically all knowledge of 

 heredity is based on the mutations that have occurred. The 

 ordinary "wild" individuals are extremely uniform; mating 

 two of these, the progeny are uniform and there is little 

 opportunity for the study of the rules of inheritance. But 

 when a gene becomes mutated, and the individual carrying 

 jt is mated with another in which that gene is not mutated, 

 the descendants display all the rules and proportions of 

 Mendelian heredity. In this way the course of heredity for 

 hundreds of structural and physiological characteristics has 

 been worked out; in each case there has been a mutation in 

 a gene affecting the characteristic. 



The same single gene becomes mutated in different indi- 

 viduals in different ways, so as to give different characteris- 

 tics in each case. So, in Drosophila there is a gene located 

 near one end of the X-chromosome, which cooperates with 

 pther genes in producing the color of the eye. If this particu- 

 lar gene and the others that work with it are in their usual or 

 "normal" condition, the color of the eye is red. If this gene 

 is mutated in a certain way (the other genes remaining un- 

 changed), the eye color changes to white. Other mutations in 

 this same gene give other eye colors, and in this way, by differ- 

 ent mutations of this single gene a whole series of eye colors 

 have been produced, some eleven or twelve in all. These eye 

 colors, resulting from different mutations of this one gene, 

 have been given the following names, beginning with those 



