lOO GENETICS 



Body structure: A defect in X produces Irregularities in 

 the abdomen, known as "abnormal abdomen." 



Legs: A defect in X-chromosomes causes abnormal de- 

 velopment of the legs; some of them are wholly or partly 

 reduplicated, so that the total number of legs is increased. 



Bristles: Modifications in X-chromosomes result in vari- 

 ous different changes in the bristles that are scattered over 

 the body. 



Many physiological conditions and functions likewise de- 

 pend upon the X-chromosomes, since they are changed when 

 these chromosomes are modified. Among these are the fol- 

 lowing: 



Positive reaction to light. Drosophila with normal X- 

 chromosomes fly toward a source of light. Those having 

 defective X-chromosomes of the kind that produce a tan- 

 colored body do not fly toward a source of light. 



Weakness and short life. Most of the defective X- 

 chromosomes that cause structural or other changes in the 

 body (different eye colors, wing forms and the like) pro- 

 duce likewise weakness and short life. The individuals bear- 

 ing them are less resistant to bad conditions, and live for a 

 shorter time than the individuals that bear the normal 

 X-chromosomes. 



Life and death. Some X-chromosomes have defects that 

 are so severe that the individuals bearing them will not live 

 and develop, unless there is present also a normal X- 

 chromosome. Such defects are known as lethals. The pres- 

 ence of such lethal defects in the X-chromosome has the 

 result that males bearing them die, since they have but one 

 X; while females usually live, since they have an additional 

 X that is often normal. If a mother that has one lethal X 

 and one normal one is mated to a father that has a normal 

 X, the result is that half of the sons which receive the 

 mother's defective X fail to develop, while the other half 

 of the sons, receiving the mother's normal X, live and 



