290 PHYSIOLOGICAL GENETICS 



allelomorph. (In ordinary mutation, however, it is the other 

 way round, and reverse mutations occur, spontaneously or 

 induced, less frequently.) The change is further favored by the 

 environment and may occur only in certain tissues, and it is also 

 favored by the presence of definite genes. These facts induce 

 Demerec to assume, as Correns did, that the gene is a single 

 organic molecule, complex, of course, but in general constitution 

 not different from known larger molecules. We have mentioned 

 the case studied by Demerec in Drosophila virilis, the unstable 

 gene for Miniature wing, which forms a multiple allelomorphic 

 series, two members of which are unstable. No change from one 

 allele into another occurs; but within an unstable line, changes 

 in the degree of stability are frequent. From this Demerec 

 draws the surprising conclusion that changes from one allele 

 to the other are independent of each other and arise by changes in 

 different groups of a gene. As so many changes of a gene are 

 lethal, and as lethals may reasonably be laid to the actual loss or 

 inactivation of the gene, it might be concluded that only very few 

 changes in the gene molecule can be tolerated. The more or less 

 considerable instability of the gene would then mean the presence 

 of a chemically unstable group. Experiments in treating such 

 unstable genes with X rays or high temperatures had negative 

 results (see, however, Gowen and Gay, 1933), from which it is 

 not concluded that these unstable genes are different from 

 ordinary mutations but that the effect of treatment is too small 

 as compared with the normal rate of change to be detected. The 

 author thinks that the logical conclusion to draw from these 

 facts, as compared with all the other facts, is that unstable genes 

 are a different phenomenon from ordinary mutation, as was 

 held by Correns, who called them sick genes. In a general way, 

 one might add that conclusions from the cases here reported 

 upon the normal phenomenon of mutation could be drawn safely 

 only from facts common to both phenomena and not from those 

 in which they differ. 



There are a number of facts that show that the problem of 

 unstable genes is far from being solved and that it is quite proba- 

 ble that a phenomenon sui generis is involved that might lead 

 to important information regarding the gene and its action. The 

 cases in question would deserve a much more complete analysis, 

 especially with the addition of embryological experimentation. 



