CHARACTERISTICS: RELATION TO ENVIRONMENT 243 



to be due to a defect in a certain gene of the X-chromosome, 

 so that it shows sex-Hnked inheritance, the abnormality 

 being dominant.^ Unless that particular defective gene is 

 present the abdomen is normal. But it is dependent too on 

 the environment. Individuals with the abnormal gene, if 

 developed In a moist atmosphere, have the abnormality. 

 But if those same individuals are developed in a dry atmos- 



FiGURE 50. Reduplicated legs in Drosophila, a characteristic result- 

 ing from a defective gene, taken in connection with development at 

 a low temperature. A, normal first leg. B and C, legs showing dif- 

 ferent types of reduplication. After Hoge (1915). 



phere, they have normal abdomens. To produce the ab- 

 normal abdomen, we must have first the particular kind of 

 defective gene that yields It. But second, even If that Is 

 present, we must have too the moist atmosphere for de- 

 velopment; without It the abnormality is not produced, 

 whatever the gene. 



Another character in the fruit-fly that acts in a similar 

 way is what Is called reduplicated legs (figure 50). If a 

 certain defective gene is present In the X-chromosome, the 

 animals show a tendency to produce double legs, or even 

 triple or multiple legs, in place of single ones.^ But this 

 does not happen unless the young develop at a low tem- 

 perature. To produce reduplicated legs, then, there must be 



