390 AN INTRODUCTION TO MODERN GENETICS 



the chromosomes are lying in contact with one another (in the way 

 indicated in Fig. 43) or on the other hand two chromosomes lying some 

 distance apart may be broken and subsequently unite in a new con- 

 figuration. If the first alternative is true, it is possible to imagine that 

 a single ionization may be sufiicient to produce a rearrangement, but 

 on the second alternative, where the chromosomes he some distance 

 apart, at least two ionizations would be necessary. As far as it is known, 

 however, the curve relating the frequency of breaks to dosage does not 

 follow the course predicted either for single or double ionizations, but 

 lies somewhere between the two. It is likely, therefore, that both 

 mechanisms of rearrangement may occur. 



One class of rearrangement has a somewhat special theoretical 

 importance. These are the very small rearrangements, which, if they 

 are not detected cytologically, may be mistaken for gene mutations, 

 It is highly probable^ for instance, that many of the so-called lethals 

 produced by X-rays (and spontaneously) are actually small deficiencies. 

 It was at one time hoped that the dosage-frequency relation would 

 make possible a distinction between these and true gene mutations, 

 but recent evidence^ shows that the small aberrations are apparently 

 produced by single ionizations and therefore cannot be distinguished 

 in this way. The mechanism by which a single ionization produces 

 an effect of the size of a minute deficiency (i.e. some hundreds of m/x 

 at least) is unknown. 



4. Rates of Single Mutations 



It is not possible to assess accurately the total mutation rate of a 

 single locus, since we can never be certain of identifying all the allelo- 

 morphs; mutations with small effects may be missed altogether, and 

 lethals can only be identified with great labour. It is, however, fairly 

 certain that some loci are more unstable than others. 



More exact results can be obtained if we investigate the frequency of 

 particular mutation-steps, that is to say, the frequency with which a 

 certain gene A mutates to a particular allelomorph A' . The first impor- 

 tant point which has been discovered is that although the mutation 

 rate from ^ to ^' is usually different from that from A' to A, they may 

 in some cases be of the same order of magnitude.^ (Mutation from the 

 wild type to a mutant form is often spoken of as forward mutation, that 

 from the mutant to the wild type as back mutation.) The mere fact that 

 a hypomorph mutant can mutate back to the wild type shows that the 



^ Belgovsky and Muller 1938, Muller 1938. ^ Timofeeff-Ressovsky 1932. 



