380 AN INTRODUCTION TO MODERN GENETICS 



within and always associated with the larger particle whose size we 

 have been estimating. We shall discuss this possibility further on 

 page 401. (For a discussion of the "sensitive volume" of the gene, see 

 P- 391). 



D. GENE MUTATION! 



The word mutation is used to cover any sudden change in hereditary 

 constitution, and is therefore applied to the origin of chromosome 

 rearrangements or of polyploidy as well as to gene mutation. We shall 

 here consider mainly gene mutation, which seems likely to provide one 

 of the most fruitful Hnes of approach to the problem of the nature of 

 the gene. 



I . Spontaneous Mutation 



Gene mutations may occur spontaneously, that is to say, with no 

 apparent cause. The total frequency of spontaneous mutation in a 

 species cannot be stated accurately, since there may be many small 

 mutations which easily escape detection. The total rate of easily detec- 

 table spontaneous mutations (including lethals) in the X chromosome 

 of D. melanogaster is about • i per cent per life cycle (i.e. about one 

 gamete per thousand bears a mutation). Allowing for the other chromo- 

 somes, and doubling the figure to allow for undetected mutations, one 

 may guess the total mutation rate at about 2 or 3 per cent.^ In other 

 organisms, the facts are even less known. There seems no reason why 

 the mutation rate should not be of the same order of magnitude in 

 terms of life cycles, but it certainly cannot be so in terms of absolute 

 time units, since if it were practically every gamete of a long Hving 

 form such as an elephant or Sequoia would carry one or more new 

 mutations, which is certainly not the case. In what follows, mutation 

 rates will always be quoted in terms of hfe cycles unless the contrary is 

 expHcitly stated. 



The spontaneous rates of individual steps of mutation in Drosophila 

 are usually of the order of o • 0005 per cent. In man, Haldane^ has calcu- 

 lated the mutation rate from the normal allelomorph to the haemophiha 

 gene as about i in 50,000. In all well-investigated organisms, there 

 are, however, great differences in the mutabiUties of different loci and 

 even of different allelomorphs. At one extreme are the so-called mutable 



1 General reference: Timofeeff-Ressovsky 1937. 



- Timofeeff-Ressovsky 1937. ^ Haldane 1935. 



