RADIATION AND THE STUDY OF MUTATION IN ANIMALS 1227 



THE STUDIES OF GENERAL MUTATION RATE 

 THE RELATION BETWEEN LETHAL AND VIABLE MUTATIONS 



In the preceding sections it has been shown that the population of 

 genes varies in the frequency with which its members mutate, that this 

 mutation is a highly localized process, and that at any given locus it 

 occurs in directions whose frequency may be related to the stability of the 

 resultmg genes. Moreover, in any group of mutations occurring under 

 conditions where chromosome breakage is frequent, many will be associ- 

 ated with the points of breakage; and the occurrence of one mutation 

 in a gamete probably increases the chance that another will be present. 

 These are the properties which must be expected to influence the composi- 

 tion of any collection of mutations to a given phenotype. It is with 

 such collections of mutations that the greater part of the mutation 

 experiments are concerned, particularly those having to do with the 

 mode of action of radiation. 



The lethal-mutation experiments, the technique of whose performance 

 has been described, make it possible to obtain measurable percentages 

 of mutation, even "spontaneously." They justify therefore, only as a 

 matter of convenience, the difficulty of interpretation introduced by 

 lumping so heterogeneous a group of genes as are necessarily collected in 

 the lethal mutations. It has long been suspected, moreover, (Morgan, 

 Bridges, and Sturtevant, 93) that these are in the main due to losses of 

 small sections of chromosomes. The question is one which needs 

 investigation in the chromosome of the sahvary glands, where alone it 

 should be possible to decide whether most lethals are actually deficiencies. 

 It is obvious, and will be considered in more detail later, that such a 



Table 8. — Comparison of Frequency of Lethal and Viable Mutations in 

 THE Sex Chromosome of Drosophila melanogaster 



Authority 



Muller (105) 



Timofeeff-Ressovsky (178). 



Patterson (138) 



Gowen and Gay (52) 



Timof 6efiF-Ressovsky ( 1 85 ) . 



Number 

 lethals 



Number 

 viables 



89 

 138 



89 

 57 



82 



112 

 320 

 302 



39 

 16 



8 

 3 

 4 



16 



44 



31 



Ratio 

 lethal: viable 



2.23* 



8.62 



10.12 

 19.00 

 20.5 



7,00 



7.27 



9.74 



* This low ratio may be due to the inclusion of autosomal dominants in the viable mutations. 



