GENE MUTATIONS 45 



showing the deepest colors: red, coral, blood, cherry, eosin, 

 apricot, ivory, tinged, buff, ecru, white, and ultra-white. 



Other genes in Drosophila and in other organisms, are 

 known thus to have been changed by mutation in many dif- 

 ferent ways; many series of modifications of a single gene, 

 like that just listed, are known. 



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 detect- 

 able way, from red to "coral" or "blood." Based largely on the 

 so-called mutations of De Vries — which were in fact mainly 

 the result of gross chromosomal irregularities — there has been 

 a prevalent tradition that mutations are sudden extensive 

 changes, "saltations," like the change from red eye to white 

 eye. It was natural that the first mutations (Observed should be 

 these very conspicuous changes: sudden loss of wings or of 

 eyes ; or other marked changes in structure. Such conspicuous 

 mutations are indeed not infrequent. 



But as studies became more exact and detailed, it was 

 found that many mutations produce extremely minute 

 changes, not to be detected without great care. For example, 

 there are known in the fruit fly a number of gene mutations 

 whose only discoverable effect is to cause a very faint lighten- 

 ing of the color of the eye in case the individual containing 

 them has eosin-colored eyes. If the eyes are not eosin-colored, 

 these mutations have no discoverable effect. The number of 

 gene mutations having such very slight effects is much 

 greater than that of those having marked effects ; most muta- 

 tions produce almost imperceptible changes. 



The majority of mutations are recessive in heredity. That 

 is, if in one of the pairs of genes, one of the two has been 

 mutated, the other not, the mutated gene produces no mani- 



