THE INTERACTION OF GENES: THE EFFECTS 165 



genes other than shaven; in the triploid a given dose of shaven has less 



brisde-producing eflfect than does the same dose in the diploid. The 



rest of the genotype therefore works in a way opposed to shaven and 



the phenotype is the result of a balance between the two. 



Hypomorphs can be deteaed by less comprehensive experiments 



than these by observing the effects of deficiencies for them. If a gene is 



working like the standard, but less efficiendy, a deficiency should work 



standard 



less efficiendy still and the heterozygote -—r-. niay show the 



defiaency 



recessive phenotype ("pseudo-dominance"); indeed, if the gene is 



nearly as efficient as the standard, the substitution for it of a deficiency 



may cause the heterozygote to show even greater difference from the 



Haplo-IV Diplo-IV Diplo-IV Triplo-IV Tetra-IV 



SY SV/SY SV/+ Sv/sy/sY Sv/sv/sv/sV 



Diploid .. extreme shaven shaven wild type slight shaven extra bristles 



Triploid .. dies extreme shaven slight shaven shaven slightshaven 



Fig. 79. Different Doses of the bristle-reducing gene shoYen (IVth chromosome) 

 in diploids and triploids of Drosophila. {After Schultz.) 



normal than does a homozygote hypomorph. This phenomenon is 

 known as exaggeration. 



b. Amorphs are an extreme case of hypomorphs. A gene may have 

 very little of the same effea as the standard gene, and the limit of 

 variation in this direction is for the gene to have no effect; these are 

 amorphs. Additions of them would cause no alteration in the pheno- 

 type, and the heterozygote standard/deficiency would appear just like 

 the normal heterozygote, and show no exaggeration. They are the 

 contraries of neomorphs (see below). 



c. Hypermorphs are genes which do the same thing as the standard 

 but do it better. Since this classffication is really a classification of 

 relations between genes, the hypermorphic relations, being the con- 

 verse of the hypomorphic one, must obviously be included. Cases such 

 as Timofeefif-Ressovsky's^ mutation (caused by X-rays) from the hypo- 

 morph forked to the wild (hypermorph) forked allelomorph demon- 

 strate the actual possibility of such genes occurring. But in organism 

 such as Drosophila when the wild type is taken as the standard, hyper- 

 morphs are rarely found. Muller gives Abruptex as the only case for 

 Drosophila. The reason for this is discussed below (p. 297). 



d. Antimorphs are genes which have an effea opposite to that of the 



^ Timofeeff-Ressovsky 1932. 



