THE INTERACTION OF GENES 



gene and the daughter it produces in its own reproduction. Gene 

 reproduction makes use of the special mechanism, we might almost 

 say of the midwife molecule, of nucleic acid. But nucleic acid appears 

 not to enter into the production of other molecules by the genes. 

 We should not, therefore, expect the same simple correspondence 

 between the gene and its effect in action, as between the gene and 

 its daughter in reproduction. Furthermore it is hardly likely that 

 genes will fail to interfere or react with one another, either in the 

 consumption of materials or in the release of products. On the 

 contrary, we should expect genes to interact with one another, 

 both within the nucleus and outside it in the cytoplasm. 



One type of interaction between genes in their work we have 

 seen in the position effect. This effect not only shows interaction; 

 it shows that the interaction can occur within the nucleus. Another 

 interaction that is probably within the nucleus is that whereby one 

 gene modifies the rate of mutation of another. In Drosophila this 

 happens at times as a result of a position effect. A gene of normal 

 stability is moved close to the heterochromatin and becomes highly 

 mutable, so much so that it regularly produces a mixture or mosaic 

 of mutant and non-mutant tissues, such as that shown by Plum-eye. 

 The control of mutability of one gene by others is common, and, 

 though not generally acting by position effect, the modification of 

 mutation rate may have something to do with the heterochromatin. 

 For example, in maize the Dotted gene, which is recognized by 

 its effect on the mutability of an anthocyanin gene, is shown by 

 linkage tests to lie in or near the heterochromatic end "knob" of 

 chromosome lo. 



Mutability control being so widespread we should expect that an 

 upset in the genetic organization or balance would be reflected in 

 changed mutation rates. It is, in fact, often claimed that F^ species 

 crosses show a greater frequency of mutation — visible somatically — 

 than either parent. 



The first types of genie interaction were recognized by Bateson 

 in the course of the series of studies between 1900 and 1 910 in which 

 he estabhshed the general validity of mendelism. These interactions 

 proved to be ones which cannot be traced back to the nucleus, and 

 indeed probably arise very far away from it. They are the types 

 which reveal themselves in the segregations of whole individuals in 



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