382 RADIATION BIOLOGY 



matin, and an increase in the abnormality of functioning of any in the 

 heterochromatin which may have been affected. Subtraction of hetero- 

 chromatin exerts, as expected, the opposite influence on these position 

 effects. 



Interpretations of the mechanism of operation of the position effect are 

 thus far speculative. One hypothesis, proposed by Sturtevant (1925), 

 has been that the effect is produced by changes in the local concentration 

 of gene products, on the assumption that the products of a gene are more 

 concentrated in its neighborhood, and are likely to react to a greater 

 extent with those of another gene when that other gene is nearby and 

 hence has its products also more available for the interaction. Thus the 

 removal or juxtaposition of the latter gene would affect the amount of 

 interaction and hence the expression of any characteristic of the 

 organism which depends on that interaction. According to another 

 hypothesis, proposed by the present writer (1935c, 1941), a gene influ- 

 ences another in Drosophila by subjecting it to localized physical forces of 

 stress and strain of the same nature as those which cause genes of like 

 composition to be drawn together in synapsis. But, when the genes are 

 of unlike composition, as would usually be true of neighboring genes, 

 these forces are exerted unequally and asymmetrically on their different 

 parts, so as to mutually influence the shapes of the genes, and with this 

 change in shape would go a change in the type or intensity of the chemical 

 activity of the gene, just as happens when protein molecules are subjected 

 to folding or unfolding. 



A distinct case of position effect on genes at two different loci has been 

 found and analyzed by Gatcheside (1939, 1947) in Oenothera hlandina. 

 The effect on both genes here is of the variegated (mosaically expressed) 

 type which in Drosophila is, as has been mentioned, characteristically 

 exerted on genes of euchromatin by heterochromatin placed in their 

 neighborhood. Nevertheless, the studies of structurally changed chromo- 

 somes in other organisms than Drosophila are on the whole conspicuous 

 for the absence of evidence of position effects. If they were nearly as 

 marked and general in occurrence in most organisms as in Drosophila, 

 it would have been found in other organisms, as in the latter genus, that 

 the great majority of translocations caused lethality or sterility, or at 

 least some visible abnormality, when received from both parents. It is 

 possible that this is true in the mold Neurospora. Yet in mice, as in 

 maize and other forms in which numerous translocations have been 

 worked with, this is certainly not the case ordinarily. The relative 

 uniqueness of Drosophila in this respect seems to lend greater plausibility 

 to the view that the cause of the position effect lies in the same influences 

 as those which bring about synapsis. For Drosophila, along with most 

 other species of Diptera, differs from most other organisms in having 

 these synaptic influences relatively strongly expressed, not merely at 



