1 64 



AN INTRODUCTION TO MODERN GENETICS 



belonging to the same locus. The choice of a standard is obviously the 

 wild-type, when such a thing exists, and it will be easiest to give the 

 definitions in terms of such a scheme, which may be appHed to an 

 organism such as Drosophila. The classification is, however, always a 

 classification of relations between genes, not of genes themselves, and 

 this must be explicitly stated for organisms in which no standard or 

 wild type is available. 



a. Hypomorphs are genes which do the same thing as the standard 

 gene, but do it less efficiently. An increase in the number of a hypo- 



'^ J- 



DOSE 



Fig. 78. The Relation between the Dose of a Hyponnorph Gene and its 

 Effect. — The locus concerned is that of bobbed in Drosophila melanogaster. Five 

 allelomorphs were used, of which two, bb' and bb" lay in the Y chromosome and 

 could, therefore, be added to flies without any difficulty. All the allelomorphs 

 are concerned in the formation of bristles, but with different efficiencies. If one 

 takes an arbitrary value of 30 for the minimum dose which produces full-sized 

 bristles, the efficiencies of the allelomorphs can be estimated as: + ^ 30, 

 bb' =10, bb = S, bb" = 4, bbl = 2. The curve shows the effects of various 

 combinations of genes in diploid females. Note that normal bristles can be obtained 

 by adding sufficient mutant allelomorphs, e.g. bbbbbb"bb' = 30, bb bb bb' bb' ^ 36, 

 etc. 



(After Stern.) 



morphic gene in a constant genotypic milieu (e.g. by adding chromo- 

 some fragments containing it and only a few other genes^ or by adding 

 otherwise "empty" Y chromosomes)^ increases the effect of the gene 

 until eventually sufficient of the hypomorph may be present to give the 

 same effect as the standard. 



Schultz^ worked with shaven, an apparently bristle-reducing (that is, 

 actually an inefficiently bristle-producing) recessive gene in the IVth 

 of Drosophila melanogaster. He could add IV's, which contain few 

 genes and can be taken as nearly simple additions of shaven, until the 

 diploid genotypic background with four doses of shaven showed extra 

 bristles, that is, the four shavens gave more effect than the wild type 

 gene. He also investigated the effect of increasing the dosage of the 



^ Muller 1933. - Stem 1929. ^ Schultz 1934. 



