136 



PR] SIOLOGICAL GENETICS 



The first systematic attempt to attack the problem of the relation 

 of gene quantity to gene action by gene addition was made by 

 Stem (19296) with the bobbed allelomorphs. The Y-chromo- 

 some of Drosophila contains the mutant gene bobbed, the norma] 



allele of which is situated in the X-cliromosomo. As hardly any 

 other genes are situated in the Y-chromosome, and as this has 

 very little effect upon development, the number of feb-genes could 

 be varied by adding more Y-chromosomes or Y-chromosome 



d 



X^fybb x b6^ibybb' 



Xbbxbbybb'ybb' 



xW^bST 



_ 



(a) 



Xbb'xbbFybb ) 'bb 

 Xbb'xbb'ybb' 



(b) 



X bb l 



X bblybb- X^YM^t^yiy 

 (C) 



L. 



H 



X bb 



(d) 



Fig. 26. — The squares on the left side: The relative quantities of the four 

 66-alleles 66', 66, 66", and 66' as derived from the length of bristles. X and Y 

 indicate the position of the alleles. The diagrams a-d: action of different combi- 

 nations of 66-alleles. Each combination shows the respective bristle, ab, female; 

 cd, male combinations. (From Stern, 1929, Biol. Ccntrbl. 49.) 



fragments. In this w f ay, males could be composed with from 

 zero to three bobbed genes, and females with from zero to two 

 such genes. Bobbed produces short bristles. In these combina- 

 tions, the length of the bristles is directly proportional to the 

 number of the 66-genes, up to the Wild-type length, which acts 

 as a threshold condition, beyond which no increase seems possible, 

 at least in this series. [As a modification by temperature action, 

 bristles of twice the length of Wild type may be produced 

 (Goldschmidt, 1935a)]. One might also express the result by 

 stating that the increase of numbers of recessive genes leads 

 toward and finally to the dominant Wild phenotype. The 

 experimental series and its result are represented graphieally 



