242 



GENETICS 



kind will be presented, selecting those that are from a tech- 

 nical point of view fully known. 



In that animal whose genetics is best known, the fruit-fly 

 Drosophila, many of the characteristics that have been 



Figure 49. Abnormal abdomen in Drosophila, a characteristic re- 

 sulting from a defective gene in the X-chromosome, but manifested 

 only when the organisms are grown in a moist atmosphere. (Wings 

 removed, to show the irregularity of the abdominal segments.) After 

 Morgan and Bridges, 19 16. 



Studied are defects or abnormalities which are sharply de- 

 pendent on the presence of particular genes. Such a one is 

 abnormal abdomen (figure 49). This appears in the fact 

 that the abdomen is ill-formed, the segments not being 

 regular nor sharply marked off. This abnormality is found 



