Chap. 20 



THE PHYSICAL BASIS OF HEREDITY 



411 



Fig. 20.20. Mutations in eyes of fruit flies (Drosophila melanogaster) induced 

 by exposure to radium. A, normal eyes, top view; B-G, different degrees of eye- 

 lessness, top views; H, normal eyes, side view; l-K, different degrees of eyeless- 

 ness, side views. (After Hansom and Winkleman. Courtesy, Fasten: Introduction 

 to General Zoology. Boston, Ginn and Co., 1941.) 



there are thousands of genes in one sperm or one egg and that the majority 

 of mutations effect such slight changes that they are not discovered. In addi- 

 tion, the majority of them are recessives that are carried in the animal but not 

 expressed for a very long time. 



On the basis of observations on fruit flies, Muller has estimated that the 

 average time clasping without change in any particular gene may be about 

 100,000 years. Allowing 10 generations of fruit flies per year, any particular 

 fruit fly would mutate only once in something like a million generations. The 

 mutation rate of the disease of hemophilia in a human line of descent has 

 been estimated by J. B. S. Haldane as one in 100,000 generations. 



Some species and some characteristics mutate more than others; fruit flies 

 have many mutations; certain colors of sweet peas and many other garden 

 flowers are mutations. 



