

THE GENE 



3 X 5 



of the genes involved. Thus in such 

 cases a gene may be studied in a single 

 quantity in comparison with the double 

 quantity in homozygous condition and 

 the plus or minus-double quantity in 

 normal heterozygous condition. The 

 source of error consists in the presence of 

 a few other genes in the same condition. 

 Bridges noticed first and other writers, 

 notably Mohr, obtained the same result, 

 that characters produced by a single gene, 

 the partner being totally absent, appear 

 exaggerated: characters produced by only 

 one gene a differ still more from the normal 

 type than the same characters produced 

 by aa. Bridges' explanation of this 

 phenomenon by genie balance is well 

 known. If according to our views the 

 decisive point is the quantity of the gene, 

 an explanation of this phenomenon should 

 be found when multiple allelomorphs are 

 involved in the same experiment. If 

 multiple allelomorphs are different quanti- 

 ties of the same gene complete absence of 

 one gene is to be regarded as the end point 

 of the series. Mohr ('27) was able to 

 study such a series, the well-known white 

 series of eye colors in Drosofhila, in the 

 case of deficiency. If white, the lowest 

 member of the series, is combined in 

 heterozygous condition with the other 

 members, it has a diluting effect on the 

 phenotype. If the members of the series 

 are combined with deficiency (viz. Wo, 

 W x o, W 2 o instead of Ww, Wiw, W^w^) the 

 effect is still further dilution. Total 

 absence of w therefore acts like a further 

 still lower member of the series. The 

 present writer feels unable to draw any 

 other conclusions from such facts than 

 that we see here the effects of a different 

 quantity of the gene. In a paper read at 

 the International Genetics Congress, 192.7, 

 Mohr produced new facts, which he 

 thinks are in favor of our conclusions. 

 It is obvious that under these circum- 



stances the phenotypic effect called exag- 

 geration has to be understood in the 

 same way as the effects of other different 

 multiple allelomorphs, namely through 

 the medium of different velocities of 

 reaction. How this can be done in detail 

 has been discussed in the present writer's 

 book ('2.7a, p. 78 ff.). 



The next possibility of studying different 

 quantities of a gene without involving 

 all genes is in trisomic forms as notably 

 given in Blakeslee's work on Datura and 

 the triplo-IV form oiDrosophila (Bridges). 

 Here of course the causes of error are 

 greater, because- whole chromosomes are 

 involved with the quantitative disturb- 

 ance of all their genes and a possible 

 general effect (nucleo-plasmic ratio) in 

 addition. In the case of Datura more 

 general results of the trisomic constitution 

 have been studied than effects of the three 

 quantities of individual genes. But in 

 the triplo-IV Drosophilas the effects of 

 three quantities of a gene could be studied. 

 Judging from the case of deficiency and 

 haplo-IV with exaggeration, the respec- 

 tive phenotype in the triplo-IV is expected 

 to be exaggerated in the opposite direction 

 than in the deficiency case, because three 

 quantities of the gene might again be the 

 same as a higher multiple allelomorph. 

 In fact these are Bridges' results, which 

 however are interpreted differently by 

 him, namely by genie balance (see later). 

 At this point a word ought to be inserted 

 about a closely related phenomenon. 

 According to the workers on Drosopbila 

 many dominant mutations in that form are 

 lethal, when homozygous; homozygous 

 deficiency is lethal; absence of both IV- 

 chromosomes is lethal; on the other hand 

 four IV-chromosomes are also lethal. 

 There is probably a consensus of opinion, 

 that in all these cases some balance in the 

 cooperation of the genes, or expressed 

 more correctly in the effect of the genes, 



