The Source of Variability 67 



suits in a tandem triplication of region 16A which in its homozygous 

 condition produces an extreme bar condition called Ultrabar. 



POSITION EFFECTS 



In some instances the same alleles have a different effect if they 

 occur in different positions. An example concerns the Bar and 

 Ultrabar alleles described above. Female flies heterozygous for Ul- 

 trabar contain the same number of Bar units as those homozygous 

 for Bar, four per nucleus, and hence might be expected to have 

 similar phenotypes, but this is not so. The number of eye facets 

 is definitely lower in the heterozygous Ultrabar. It appears that 

 in this case the extra Bar regions are more potent in their effect 

 when situated on the same chromosome. This exemplifies position 

 e-ffect, to wit, that the same units produce a different genetic effect 

 in different positions. 



SUPPRESSION EFFECTS 



Blum (1951) pointed out that mutations should be the result of 

 a chemical reaction, that this reaction should be reversible to some 

 extent, and that as a result mutations should occasionally mutate 

 back to the original condition. Apparent reverse mutations have 

 indeed been recorded on many occasions. Stadler (1955) warned, 

 however, that observed phenotypic reversions may be due not to 

 a reverse mutation but to an inhibitor gene which blocks the ex- 

 pression of the mutant effect, although the mutant gene is still 

 present. Glass (1957) gave two interesting examples of such an 

 effect concerning mutant alleles in Drosophila which produce ab- 

 normal adult eyes or tumors in the larvae. Glass and his colleagues 

 investigated the biochemical pathways involved in the expression 

 of the alleles in this genetic complex and concluded that so-called 

 "suppressor" genes may be channelling agents for substrates needed 

 for the expression of many genes rather than being modifiers of 

 any particular gene under study. In this situation, apparent sup- 

 pression of a gene would be the suppression of its effect because 

 of competition for substrates. 



ENZYMATIC EFFECT 



Investigations of the mold Neiirospora by Beadle and Tatum 

 (Beadle, 1945) and of the bacterium Salmonella by Zinder and 

 Lederberg (Zinder, 1953) demonstrated the occurrence in these 

 species of alleles whose absence resulted in the inability of the 

 organisms to synthesize specific essential chemical materials such 



