THE MUTATED GENE 151 



There follow now the cases in which one chromosome may be 

 obtained in simplex, duplex, and triplex condition, without 

 regard to individual mutant genes. Such is the case, for exam- 

 ple, for the fourth chromosome of Drosophila (Bridges). As 

 this is small and supposed to contain few genes, the effects ought, 

 to be of a comparatively good dosage type. A haplo-IV fly is 

 distinguished from a normal one by smaller body size; shorter 

 and more slender bristles; paler body color; darker thoracic 

 pattern; larger and rougher o\(^; slightly spread, blunt, and 

 cloudy wings. A triplo-IV fly, containing three fourth-chromo- 

 somes, shows small and smooth eyes; narrow, more pointed wings; 

 darker body color; suppressed trident pattern. In other words, 

 the three dosage types 1,2,3 fourth-chromosomes form an orderly 

 series of quantitative expression of a number of phenotypic 

 traits in the order subnormal-normal-hypernormal expression. 

 It may be added here that the presence of the mutant gene 

 eyeless in this chromosome leads to a comparable dosage effect 

 for this gene alone. 



Other examples of the same type may be found in Blakeslee's 

 (since 1921) work on Datura. The interpretation in terms of 

 velocities of gene-controlled reactions is the same as given above 

 for the action of individual genes. The difference is that there 

 only one or a few interwoven reactions were changed in their 

 time relations, and correspondingly some special somatic traits 

 were shifted proportionally to dosage and affected reaction 

 velocity. In the case of whole chromosomes, which are' probably 

 concerned with many, and therefore decisive, reactions, general 

 developmental features are affected, and therefore abnormalities 

 of practically the whole animal will result. In plants, the 

 phenotypic expression of such a dosage change will appear 

 predominantly as a change of the whole habitus which becomes 

 visible at once and which might be described in quantitative 

 terms for each discernible character. It is important to mention 

 the fact, first discovered by Blakeslee in Datura and later found 

 in other plants, that the phenotypic effect of a triplex condition 

 of one chromosome is typical and different for each chromosome, 

 showing that the genes affecting general developmental processes 

 like size, type of branching, and growth type are not distributed 

 at random among the chromosomes. 



