l8o GENETICS 



cause a change in the characteristics of a number of different 

 parts. Some examples of this may be given. 



The character bent wings, belonging to linkage Group 

 IV of Drosophila, is due to the modification of a single gene 

 in chromosome IV. It causes the wings to be bent dorsally 

 at their base, and also it causes the legs to be shorter than 

 normal, owing to the abnormal shortness and thickness of 

 the metatarsal joint. 



In animals showing the character vestigial wings (due 

 to modification of a gene in chromosome II), the following 

 peculiarities appear: the wings are degenerate, only their 

 bases being present, and these are held at right angles to 

 the body. The balancers are likewise degenerate. Two rear 

 bristles on the scutellum are farther apart than usual, and 

 they stand up straight instead of inclining backward. The 

 flies hatch about two days later than in the normal wild 

 flies. They are not quite so vigorous as the normals. 



As noted in the preceding chapter in connection with 

 linkage Group I, modification of a certain gene in the X- 

 chromosome causes ( i ) the body color to be tan instead of 

 gray, and (2) causes the individuals to lose the usual posi- 

 tive reaction to light. 



In any detailed account of the effects of particular genes, 

 it will usually be found that each gene has thus several 

 different effects, often on diverse parts of the body. The 

 effect on which the name given to the gene is based is com- 

 monly merely that effect that is most conspicuous to the eye. 



3. General or Constitutional Effect of a Gene: In addi- 

 tion to the special effects on certain structures or functions, 

 modification of any gene has also a general effect on the 

 bodily constitution. In Drosophila the "wild-type" flies, with 

 unmodified genes, are vigorous and long-lived. Practically 

 all modifications of the normal genes make the animals less 

 vigorous, so that individuals showing any of the character- 

 istics listed in the preceding chapter are weaker than the 



