342 Action of the Genetic Material 



and the two-dose types to be compared, the sexes, differ otherwise 

 in their developmental systems. Disregard of this fact has led to 

 what I consider serious errors (to be discussed below). Another type 

 of dosage difference is available when a mutant locus is located op- 

 posite a deficiency and thus can be appraised in a one-dose condition. 

 Here the difficulty is that we know now that a deficiency may have a 

 position effect, with the result that a deficiency break near the locus 

 may act like a mutant, thus restoring an effect equal to that of two 

 doses. With larger deficiencies this is avoided. The third type of 

 dosage experiment permits studying three and four doses of a locus, 

 when it is possible to duplicate it by appropriate translocations of 

 small chromosomal sections. When these are suflBciently small and the 

 results congruent, we may safely disregard the extra material present. 

 Less certain are the results when dosages are produced by dupUcation, 

 triplication, and so on, of one chromosome, which affects the entire 

 collaboration of developmental reactions, the so-called genie balance. 

 More rehable are the results in triploids, and so on, though we are 

 not sure what the effects of the physiological changes attached to 

 polyploidy are in each case. Even completely balanced triploids, as in 

 Drosophila, are usually not normal flies. 



Before using the results of all such procedures to draw conclu- 

 sions concerning genie action, we may consider the possibilities. We 

 have already mentioned the first possibihty: genie action of a qualita- 

 tive type, with one mutant locus affecting one inhibition of a synthetic 

 step. With such actions no dosage effect could be expected. A second 

 type may be described as an inhibition of a normally occurring proc- 

 ess, not as an all-or-none effect but as a quantitative effect. This may 

 be the lowering of the rate of reaction of a process needed for normal 

 development, or the lowering of the speed of growth and cell divisions 

 needed for the normal result, or any such quantitative inhibition which 

 is not of the all-or-none type. The result will be that increase of 

 dosage of the mutant locus increases the degree of abnormality of the 

 mutant. A third possibility is that the mutant locus interferes quanti- 

 tatively with the production of a substance that is needed for some 

 developmental feature in such a way that the mutant locus produces 

 the necessary stuff, but not in suflBcient quantity. Again various de- 

 tailed effects may result, as long as they are only quantitative devia- 

 tions from normal. The effect of this type of action would be that one 

 dose gives only the extremest mutant effect, which becomes less ex- 

 treme with increasing doses up to normal when a certain saturation 

 point is reached. 



