512 - Heredity and Evolution 



been produced. Mutations may also occur in 

 the somatic cells of the organism, but in this 

 case the effects are usually limited to some 

 particular part or parts of the body; and 

 such changes are not perpetuated in subse- 

 quent generations. Thus, a somatic mutation 

 usually produces a mosaic organism, such as 

 a drosophila having a red eye on one side and 

 a white eve on the other. 



The phenotypic change induced by a mu- 

 tant gene may be so slight that it can be de- 

 tected only bv extremely careful study, or, 

 the change may be drastic enough to kill the 

 organism. On the basis of the degree of 

 phenotypic change (induced by the mutant 

 when it is homozygous), mutations are usu- 

 ally classified as (1) slight mutations, which 

 can be recognized only by statistical analysis 

 and other special methods; (2) visible muta- 

 tions, which can be recognized at a glance; 

 and (3) lethal mutations, which are so dras- 

 tic that the homozygous organism dies before 

 reaching sexual maturity. 



Mutation Frequencies. Rare though they 

 are, mutations are not so rare as was first 

 supposed. // all types arc considered. In Dro- 

 sophila mclanogaster, some 500 different 

 visible mutations were distinguished in sur- 

 veys covering 25,000,001) individuals of 

 known genetic stock; that is, about 1 fly in 

 50,000 plainly showed a mutant quality. And 

 when special methods were employed for the 

 detection ol lethals, (his type of mutation 

 was found to occur about six times more fre- 

 quently than visible mutations. 



The methods for estimating the frequency 

 of slight mutations ate not exact, but many 

 experts believe that slight mutations may 

 take plate even more often than either of 

 the other types. In fact there are now- reliable 

 estimates for Drosophila that indicate that 

 between 1 and 10 percent of the gametes in 

 each generation display at least one muta- 

 tion of one type or another. In man, the esti- 

 mates indicate that each of us, on the average 

 may receive one mutated gene, via the egg or 

 sperm, at the time of conception. 



A particular mutation, on the other hand, 



will not occur, except rarely, although differ- 

 ent genes vary considerably as to their muta- 

 bility. Among certain colon bacteria, for ex- 

 ample, the mutation that makes the bacilli 

 resistant to streptomycin shows up in only 

 about one cell per billion in each generation; 

 whereas the mutation responsible for hemo- 

 philia in man appears at least once in every 

 100,000 gametes. 



The same gene may mutate successively in 

 several different directions, giving rise to a 

 series of allelic genes. In the fruit fly, for 

 example, the gene 11', the normal allele of 

 white (w) gene, has given rise to 1 1 mutant 

 alleles, each having a different effect upon 

 eye color and upon other phenotypic char- 

 acteristics. Several of these mutations have 

 been duplicated in different stocks on differ- 

 ent occasions; and in some cases the same 

 mutant has arisen from two different alleles. 

 Thus in the white (u») series, the gene w e 

 (eosin) has arisen not only from IT 7 (red) but 

 also from w (white); and eosin has mutated 

 back to white and also to red. 



As a general rule the mutant gene has its 

 chief effects upon the same part or character 

 as the normal counterpart, though the effects 

 may vary widely as to quality and intensity. 

 Except for this fact, however, mutations ap- 

 pear to be quite random in their phenotypic 

 effects; and this is probably why so many 

 mutations are lethal in result. In the delicate 

 balance of developmental and functional 

 processes in a complex organism, any con- 

 siderable change is more likelv to "gum up 

 the machinery" than merely to modify its 

 operational behavior. 



Some insight is currently being gained into 

 the mechanisms of mutation (p. 525). The 

 frequency of mutation is known to increase 

 at higher temperatures. Moreover, the mu- 

 tation rate is much higher when the organ- 

 ism is exposed to short-wave radiations such 

 as x-rays, radioactivity, and cosmic rays, and 

 when cells are treated with certain chemicals, 

 notably some of the "nitrogen mustard" 

 compounds. There is approximately a three- 

 fold increase in the frequency of mutation 



