GENETIC VARIATIONS 343 



tectable way, from red to "coral," or "blood." Based largely 

 on the so-called mutations of De Vrles — which were in fact 

 mainly not gene mutations, but the result of chromosomal 

 irregularities — there has been a prevalent tradition that mu- 

 tations 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 ex- 

 ample, there are known in the fruit-fly a number of gene 

 mutations whose only discoverable effect is to cause a very 

 faint lightening of the color of the eye, In case the individual 

 containing them has eosln-colored eyes. If the eyes are not 

 eosin-colored, these mutations have no discoverable effect. 

 The number of gene mutations having such very slight ef- 

 fects is much greater than that of those having marked ef- 

 fects. Most mutations produce barely perceptible changes 

 in characteristics. 



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- 

 fest effect on the individual; the latter remains quite un- 

 changed. Thus one or many gene mutations may have oc- 

 curred in the chain of genes of a given individual, but so 

 long as only one gene of any pair Is affected, there may be 

 no manifest effect. Only when both members of a pair of 

 genes are mutated does the recessive mutation produce its 

 bodily effect. A small proportion of the mutations that oc- 

 cur are dominant; In this case even when but one member of 

 a pair of genes has mutated, the individual Is changed by the 

 mutation. 



