SYNTHETIC SEQUENCES 



Drosophila melanogaster breaks a wing vein of the flies. At 19° C. 

 it affects all homozygotes but it is expressed in only half of them 

 at 25° C. In the heterozygote, according to Stern, it does not express 

 itself at all at 25° C. but at 13° C. it is seen in about 10 per cent of 

 the flies. Another gene, Scutenick, which is lethal in the homozygote, 

 produces multiple effects in the heterozygote at 25° C. As the 

 temperature rises its effect on the eye decreases, while its effect on 

 the scutellum increases. 



Both of these modifiable heterozygotes belong to genes in the 

 small fourth chromosome near a block of heterochromatin, whose 

 behaviour, as we saw, is sensitive to changes of temperature. They 

 are, therefore, special cases. The reason for the general lack of 

 outside effects on dominance is not far to seek. The normal or wild- 

 type gene exists as part of a genotype long adapted to give a constant 

 expression in varying environments. Now this normal and buffered 

 gene is generally the dominant. Its heterozygote would, therefore, 

 be expected to give a more stable result than would be given by its 

 new untried allelomorph when homozygous. In fact, as Fisher has 

 pointed out, the genotype, whose effects we have seen, must have 

 been selected to give stable dominance. 



Are there any general physiological principles that we may infer 

 from these actions and interactions of genes ? One most general 

 principle is at once clear. Every gene depends, for its action, on 

 that of others; of many others, possibly of all the others. The 

 phenotype as a whole depends on the co-ordination of all genes, 

 and this co-ordination, in the fit organism, is what we call balance. 

 Hence we can see why polyploidy has a balanced effect on the 

 genotype and why polysomy, duplication and single gene changes 

 have unbalanced effects. Now, in the terms of materials and processes, 

 co-ordination means that each gene is fed by others, and, in turn, 

 feeds others. How, we must ask, is their feeding adjusted? The 

 answer to this question is given by the precise chemistry of gene 

 effects. 



Synthetic Sequences: Supply and Demand 



We can frequently see what the substitution of a particular gene 

 by its mutant does to the chemical processes of the organism. A 

 recessive dwarf in maize owes its dwarfhess to a characteristic 



Elenttnts of Genetics l6l L 



