NATURE OF THE GENETIC EFFECTS 395 



those which permit considerable survival but result in some abnormality 

 readily visible to the trained observer — the visibles — is about the same, 

 namely, in the neighborhood of 7 or 8 to 1, both for spontaneous muta- 

 tions and those induced by ionizing radiation. 



Although (as was stated on p. 391) the different mutations of any single 

 gene fail to be distril)uted, as regards the frequencies of those giving 

 different amounts of effect, in anything like a "normal curve" — inasmuch 

 as those of more extreme effect are often commoner than those of lesser 

 effect — nevertheless when a given character (e.g., eye color), instead of a 

 single gene, is examined, and mutations in all the numerous genes which 

 affect that character in any degree are taken into consideration, it is 

 found that the mutations affecting that character to a lesser degree are 

 more frequent than those of greater degree (Muller, 1923). This is not 

 surprising in view of the complicated net of biochemical reactions that 

 underlie both the processes of general metabolism, those of morphogeny, 

 and those of special physiology, since it is to be expected that there would 

 be fewer genes with a strong, specialized effect on the development of any 

 given character than those which (ha-\'ing been specialized primarily in 

 relation to other characters, which are likely to be invisible) influenced the 

 character in question merely incidentally and slightly. There is indeed 

 evidence that still more frequent than the mutations with slight yet 

 appreciable effect are those the effect of which, on any visible character, is 

 below the threshold of detection by ordinary means (Altenburg and 

 Muller, 1920). This general principle holds both for spontaneous and 

 radiation mutations. 



One of the most all-embracing, generalized "characters" capable of 

 being observed and measured is the viability, i.e., the ability to survive 

 until some given point in the life cycle has been reached; in Drosophila 

 this is usually taken, for convenience, as the beginning of the reproductive 

 period. The measure of viability of a mutant then is the frequency with 

 which individuals of the given type survive to maturity, as compared with 

 (divided by) that with which nonmutant individuals do so. Those 

 mutant genes which allow no individuals at all to reach this stage are the 

 ones designated as (complete) lethals; those which have between and 

 10 per cent of the normal survival rate are for convenience distinguished 

 as "semilethals" or (a better term for them) sublethals; while those with 

 over 10 per cent but less than 100 per cent of normal viability are termed 

 detrimentals. The great majority of mutants (whether spontaneous or 

 radiation induced) which cause any kind of externally visible morpho- 

 logical abnormality are found to be in some degree detrimental, and, in a 

 general but very imperfect way, there is a tendency for a greater degree 

 of visible abnormality to be associated with a greater amount of detri- 

 ment (lower viability). This prevailingly detrimental nature of muta- 

 tions, of whatever origin, is undoubtedly a consequence of the fact (also 



