340 



MUTATIONS 



had the same number of chromosomes as their parents. There- 

 fore, it may be assumed that when the chromosome number does 



normal 

 Vising 



notcVu 



vestigial 



miniature 

 wing 



clvcb 

 vvxng^ 



ruStimentory 



truncode 



normal 



boci* 

 eve 



eyeless 



eye 



T. H. Morgan and his associates have observed the appearance of hundreds of new characters, 



mutations, in fruit flies. 



change, a mutant is produced, but that germinal variations may 

 also arise in other ways. Morgan has shown that mutations 

 sometimes arise as a result of one gene or more actually changing 

 in character. He has identified over 400 characters which are 

 carried by the eight chromosomes of the fruit fly (four chromosomes 

 in each germ cell). He believes that each character is determined 

 by a gene in a chromosome. Therefore, there are many genes in 

 each chromosome. Experimental work in heredity makes it seem 

 highly probable that these tiny units of inheritance are present in 

 the chromosome, and if they change in character it again seems 

 plausible that a germinal variation would result. 



(c) Crossing-over of genes. Several scientists have done experi- 

 mental work that would seem to show that sometimes the genes 

 actually cross over from one chromosome to another. Just be- 

 fore reduction-division, the chromosomes always arrange them- 

 selves side by side in pairs. In the fruit fly there would be four 



