342 GENETICS 



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 

 it 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, located at a definite point in the 

 genetic system, becomes mutated in different individuals in 

 different ways, so as to give different characteristics in each 

 case. So, in Drosophila there is a gene located at the point 

 1.5 near one end of the X-chromosome (figure 38) which 

 cooperates with other genes in producing the color of the 

 eye. If this particular 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 unchanged), the eye-color changes 

 to white. Other mutations in this same gene give other eye- 

 colors, and in this way, by different mutations of this single j| 

 gene, a whole series of eye-colors has been produced, some 

 eleven or twelve in all. These eye-colors, resulting from dif- 

 ferent mutations, form the series of "multiple alleles" men- 

 tioned on page 181, under the following names: coral, 

 blood, cherry, eosin, apricot, tinged, buff, ecru, ivory, white, 

 ultra-white. Other genes in Drosophila and in many other 

 organisms are known thus to have been changed in their 

 effects by mutation in many different ways, so that many 

 series of multiple alleles, or modifications of a single gene, 

 are known to exist. 



The change induced by the mutation of a gene may be 

 great, or it may be very small. The eye of the fruit-fly is 

 changed by a certain mutation from red to white, a great 

 change. By another mutation it Is changed in a barely de- 



