56 



THE AMERICAN NATURALIST [Vol.LVI 



normal fourth-chromosome. With respect to a character 

 that is exaggerated in a plus direction the fourth-chromo- 

 some has an unbalance in the minus direction. But since 

 the whole complement is in balance, this unbalance within 

 the fourth-chromosome is neutralized by a reciprocal un- 

 balance in the other chromosomes. So the removal of 

 one fourth-chromosome with its excess of minus modifiers 

 leaves the remainder of the genes with an excess of plus 

 modifiers, and these plus modifiers are free to work in 

 the same direction as the recessive gene that is present, 

 and thus to give an even greater effect than the homo- 

 zygous recesive. Corresponding to these exaggerated 

 fourth-chromosome characters there is a class of sex- 

 linked characters that are exaggerated in the absence of 

 one X-chromosome. These mutant characters show a 

 different grade of development in the male from that 

 which they show in the female. A good example is the 

 race called eosin, in which the male has a much paler eye- 

 color than the eosin female. These characters exagger- 

 ated by the absence of an X are called sex-limited. Some 

 of them, like eosin, are exaggerated in a plus direction, 

 corresponding to an excess of minus modifiers within the 

 X-chromosome, while others, such as bobbed, are exag- 

 gerated in a minus direction. Thus bobbed, which shows 

 scarcely at all in the males, corresponds to an excess of 

 genes within the X tending to make bristles short, and two 

 X-chromosomes can outweigh the genes in the autosomes 

 that tend to make the bristles long, but one X is not 

 enough to do so. 



When haploidy for the fourth-chromosome is combined 

 with mutants w^hose genes are outside the fourth-chromo- 

 some there is of course no effect corresponding to sex- 

 linkage, but there is "exaggeration." Thus, haploidy 

 for the fourth-chromosome exaggerates the third-chromo- 

 some mutant Hairless in a plus direction. This type 

 of exaggeration finds its parallel in the 20 or so sex- 

 limited mutations that are not sex-linked. These are 

 mutations whose differential genes are in the autosomes 

 and not in the X and which iicvi^rthcless show a (liff(>rent 

 grade of dovch>|nii.Mit in Ihc luah' fi-mu thnt in the Tcniale. 

 In these cases also the niodilicrs of oat-li character are of 

 different weights in the X from the general collection, 



