The Journal of Heredity 



gene and its modifiers for stability of ex- 

 pression, when under the influence of 

 environmental and genetic conditions 

 which would afifect the gene's operation 

 quantitatively, i.e. in a manner similar 

 to that of dosage changes. This does not 

 mean that selection has specifically 

 worked to produce dominance of the 

 normal gene over its alleles, however, 

 because (3) not all mutant genes be- 

 have merely like weaker normal genes, 

 and (4) those which the dosage tests 

 show to produce qualitatively difTerent 

 effects from the normal genes seem oft- 

 ener to escape from the principle of be- 

 ing dominated over by the normals, just 

 as would be expected on our hypothesis. 

 Among the further results of gene 

 dosage studies carried out by the use 

 of chromosome fragments produced by 

 irradiation, attention should be especial- 

 ly called to the findings coming under 

 the head of "dosage compensation." 

 These have shown (1) that, when the 

 dosage of virtually all genes in the X 

 chromosome except a given one is held 

 constant, the expression of that one is 

 usually so very nearly the same when 

 present in one dose as in two that no 

 difference in the character can ordinarily 

 be seen, and (2) that nevertheless this 

 invisible difference has been so impor- 

 tant for the organism that, in the 

 course of the past natural selection, 

 a system of modifying genes, called 

 compensators, has been established, 

 having the function of making the 

 effects of the one and two doses nor- 

 mally present in the two respective sexes 

 much more nearly equal still, when these 

 dosage differences in the given genes are 

 present simultaneously with those in all 

 the other X-chromosomal genes. Each 

 gene seems to have acquired a different 

 system of compensators, the interrela- 

 tions of all together being extremely 

 complicated. This then gives evidence 

 from a new angle of the meticulousness 

 of natural selection, of the very precise 

 adaptiveness of the characters existing 

 in a species, and of the final grade of a 

 character having ordinarily become es- 

 tablished through the accumulation of 

 numerous small mutations having very 



complex functional relations with one 

 another. It is in line with our previous 

 thesis of evolution through the selection 

 of multitudinous tiny accidental changes. 

 When attention is concentrated on a 

 given very circumscribed region of a 

 chromosome, by a comparison of vari- 

 ous induced rearrangements all of which 

 have a point of breakage within that re- 

 gion, other facts come to light, bearing 

 on the problems of chromosome and 

 gene divisibility. By means of special 

 genetic methods, which cannot be de- 

 tailed here, evidence has been obtained 

 that the breaks in any such limited re- 

 gion tend to occur at specific points, 

 giving indication that discrete units or 

 segments lie between these points, and 

 thus arguing against the idea of the chro- 

 mosome being a continuum and in favor 

 of its genes corresponding to physical 

 entities rather than merely to concepts 

 arbitrarily set up for the convenience of 

 geneticists. We are also enabled in this 

 way to make estimates of the probable 

 number of genes in the chromosome, as 

 well as to get maximally limiting figures 

 for their size. These estimates agree as 

 closely as could have been expected with 

 those based on previous genetic work, 

 using entirely different methods, al- 

 though not with the c^tima+es based on 

 the "sensitive volume" hypothesis. 



Duplications and Evolution 



Another finding made in studies of 

 cases having a small fragment of chro- 

 mosome moved, as a result of irradia- 

 tion, to another position, was that indi- 

 viduals are frequently able to survive 

 and reproduce even when they have the 

 given chromosome part present in its 

 orieinal position as well as in the new 

 position. In fact, it was in work of this 

 kind that the effect of extra doses of 

 genes was determined. Now, in some of 

 these cases stocks could even be obtained 

 which were homozygous for the dupli- 

 cated piece as well as for the original 

 piece. This led to the idea that duplica- 

 tions of chromosome material might in 

 this manner have become established in 

 the previous course of evolution. When, 

 in the analysis of a limited region of the 



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