Types of Mutations 225 



Mutations and Fluctuations 



Strains that are apparently homozygous sometimes produce 

 individuals different in appearance from the usual type. Often 

 such individuals result from different environmental conditions. 

 Differences, which are the direct result of changes in the en- 

 vironment, are called fluctuations. Environmental differences of 

 this sort act only on the body, or soma, of the individual and do 

 not affect the germ cells. They are therefore not inherited and 

 are of no consequence in the 

 study of genetics except to 

 show that for comparable re- 

 sults all the individuals must 

 have as nearly the same en- 

 vironment as possible. 



Sometimes, however, the new 

 types that appear may not 

 be the result of any environ- 

 mental influence and they may 



, -J. i-u • 1 J. X Fig. 70. The head of the white- 



transmit their new characters to , r r^ i -i i 



eye type oi Drosopnila melano- 



subsequent generations. Such .^^.^^^ Contrast with Figs. 6, 7, 

 nonenvironmental, heritable, and 65. 

 suddenly appearing, new types 



are mutations and are of great importance in the study of genet- 

 ics. An example of such a new form is the white eye mutation of 

 Drosophila melanogaster. In a stock of true-breeding red-eyed 

 flies, a fly with white eyes (Fig. 70) suddenly appeared for no 

 known reason. When this fly was mated to red-eyed flies, the 

 white-eye character behaved as if it were determined by a 

 recessive gene located in the X chromosome. This new type was 

 inherited and was a mutation.. Discovered by Morgan in 1910, 

 it was the first sex-linked mutation that appeared in Drosophila 

 cultures. 



Types of Mutations 



The possibility that new types may appear suddenly and 

 without any traceable cause and that they may be inherited was 

 suggested by de Vries in 1901 as the result of some observa- 

 tions on the evening primrose, Oenothera Lamarckiana. He 

 showed that this plant occasionally but regularly produces new 



