no 



stand the properties and possibilities 

 of the individual gene, we must study 

 the mutations as directly as possible, 

 and bring the results to bear upon our 

 problem. 



(a) The Quality and Quantity 

 of the Change 



In spite of the fact that the drawing 

 of inferences concerning the gene is 

 very much hindered, in this method, 

 on account of the remoteness of the 

 gene-cause from its character-effect, 

 one salient point stands out already. It 

 is that the change is not always a mere 

 loss of material, because clear-cut re- 

 verse mutations have been obtained in 

 corn, Drosophila, Portulaca^ and prob- 

 ably elsewhere. If the original muta- 

 tion was a loss, the reverse must be a 

 gain. Secondly, the mutations in many 

 cases seem not to be quantitative at all, 

 since the different allelomorphs 

 formed by mutations of one original 

 gene often fail to form a single linear 

 series. One case, in fact, is known in 

 which the allelomorphs even affect 

 totally different characters: this is the 

 case of the truncate series, in which I 

 have found that different mutant genes 

 at the same locus may cause either a 

 shortening of the wing, an eruption 

 on the thorax, a lethal effect, or any 

 combination of two or three of these 

 characters. In such a case we may be 

 dealing either with changes of differ- 

 ent types occurring in the same ma- 

 terial or with changes (possibly quan- 

 titative changes, similar in type) oc- 

 curring in different component parts 

 of one gene. Owing to the universal 

 applicability of the latter interpreta- 

 tion, even where allelomorphs do not 

 form a linear series, it can not be cate- 

 gorically denied, in any individual 

 case, that the changes may be merely 

 quantitative changes of some part of 

 the gene. If all changes were thus 

 quantitative, even in this limited sense 



MULLER 



of a loss or gain of part of the gene, 

 our problem of why the changed gene 

 still seems to be autocatalytic would 

 in the main disappear, but such a situa- 

 tion is excluded a priori since in that 

 case the thousands of genes now exist- 

 ing could never have evolved. 



Although a given gene may thus 

 change in various ways, it is important 

 to note that there is a strong tendency 

 for any given gene to have its changes 

 of a particular kind, and to mutate in 

 one direction rather than in another. 

 And although mutation certainly does 

 not always consist of loss, it often 

 gives effects that might be termed 

 losses. In the case of the mutant genes 

 for bent and eyeless in the fourth 

 chromosome of Drosophila it has even 

 been proved, by Bridges, that the 

 effects are of exactly the same kind, 

 although of lesser intensity, as those 

 produced by the entire loss of the 

 chromosome in which they lie, for 

 flies having bent or eyeless in one 

 chromosome and lacking the homol- 

 ogous chromosome are even more 

 bent, or more eyeless, than those hav- 

 ing a homologous chromosome that 

 also contains the gene in question. The 

 fact that mutations are usually reces- 

 sive might be taken as pointing in the 

 same direction, since it has been found 

 in several cases that the loss of genes— 

 as evidenced by the absence of an 

 entire chromosome of one pair— tends 

 to be much more nearly recessive than 

 dominant in its effect. 



The effect of mutations in causing 

 a loss in the characters of the organism 

 should, however, be sharply distin- 

 guished from the question of whether 

 the gene has undergone any loss. It is 

 generally true that mutations are much 

 more apt to cause an apparent loss in 

 character than a gain, but the obvious 

 explanation for that is, not because the 

 gene tends to lose something, but be- 

 cause most characters require for 



