180 GENETICS AND EUGENICS 



nature is, but it would seem improbable that any one of 

 them is itself Enzyme II, but only that it is in some way con- 

 cerned in the production of Enzyme II, either locally or 

 generally. 



As regards Enzyme I, which is produced in several grades 

 (qualitative or quantitative) through mutations in the color 

 gene resulting in multiple allelomorphs, we know that its 

 action may be localized by other independently inherited 

 genes, those not in the same linkage-group or chromosome. 

 Such are the factors for white spotting which in no case have 

 been shown to be linked with albinism. Some factors of this 

 sort seem to interfere with the production of Enzyme I in 

 particular parts of the body, others allow Enzyme I to be 

 produced but inhibit its action in particular body regions. 

 Again we have no present knowledge as to what the nature 

 of these modifying genes is. In Drosophila there occur in a 

 single linkage system (chromosome), genes affecting various 

 parts of the body and affecting them in various ways. Thus 

 in the sex-linked group of genes are found those which in- 

 fluence the shape of the eye, the color of the eye, the length 

 of the wings, the shape of the wings, the venation of the 

 wings, the form of the legs, the color of the body, the shape 

 of the bristles on head and thorax, the form of the abdomen, 

 and many others less easy of description. 



Again, in the ** second-chromosome" linkage group of 

 Drosophila, are found other genes which also affect practi- 

 cally all regions of the body, as for example, shape, size, and 

 venation of wings, length of legs, color and structure of the 

 compound eyes, patterns of thorax, shape of abdomen, and 

 general body-color. No linkage system specializes in genes 

 of any particular sort, or affecting any particular region of 

 the body. Often a single gene is known to affect various 

 parts of the body. Thus the gene, "dachs," affects both the 

 length of the legs and the venation of the wing. 



If any part of any chromosome of an egg of Drosophila 

 were removed or changed in composition, it seems probable 

 that some departure from normality would follow in the fly 



