THE MUTATED GENE 135 



difference from the cases just reported is that the homozygous 

 mutant gene has a more extreme effect than one dose plus 

 deficiency. The seriation here is from the standpoint of maxi- 

 mum abnormality: Ab/Ab > Ab > Ab/+ > +. He calls such 

 mutants, which apparently do not act in the same direction 

 as the Wild-type allele but in a different direction altogether, 

 though to a lesser degree, antimorphs. Leaving aside the question 

 of whether or not the cases ranged in this group belong actually 

 to gene mutations, it must be pointed out that the difference 

 between the two cases is not a difference of the type of mutation 

 but a difference concerning the processes controlled by the genes 

 in question, a difference in physiology of development. In the 

 case of the eye pigment, the mutant gene produces less pigment 

 or less oxidized pigment; it is deficient in action, or, more cor- 

 rectly, it controls a lower velocity of reaction; one dose therefore 

 must have the minimum pigment-producing effect. In the case 

 of the abnormal abdomen, the effect of the mutant gene is to 

 produce a situation that prevents normal segmentation or 

 normal concrescence of the segments, whatever the immediate 

 cause may be. Inspecting the different degrees of this effect, 

 we realize that the gradation of the effect is proportional to the 

 time of onset of the disturbing feature. The mutant genes then 

 control a reaction that leads to a threshold of action at a definite 

 time of development. Two doses of the gene will, just as in the 

 pigment case, produce a faster reaction than one dose. In this 

 case, however, faster reaction means earlier onset of the disturb- 

 ance and therefore more extreme disturbance. This deliberation 

 shows that the cases that Muller calls antimorphs lead to exactly 

 the same conclusions in regard to the effects of gene dosage as 

 the others, if viewed from the standpoint of genie action in 

 development. Further material bearing upon these problems 

 will be found in the next chapter. 



C. Differences of Dosage by Addition of Genes 



The elaborate knowledge of Drosophila genetics has made it 

 possible to build up individuals that contain series of different 

 dosages of one gene and thus to test further the influence of gene 

 quantity upon the product of genie action. One such case has 

 been available for a long time, though it was realized only recently 

 — the case of the Bar-eye mutants, which will be discussed shortly. 



