No. 583] ROLE OF ENVIRONMENT IN DROSOPHILA 393 



got the sex chromosome bearing the factors for red and ab- 

 normal (EAb), from her father the homologous sex 

 chromosome that carries the factors for white and nor- 

 mal (WN). 



TABLE I 



Parents: EAb $ by WN <? 

 P,: EAb $ — EAb £ 



If these chromosomes unite at synapsis without ex- 

 change of materials, half of the eggs that result (one 

 chromosome being eliminated in the polar bodies) will 

 contain the red normal combination, the other half the 

 white normal. These represent the ' ' non-cross-over" 

 gametes. If, however, these chromosomes should cross 

 and reunite, as in the diagram (the crossed lines indi- 

 cate where the crossing over may occur, not how it oc- 

 curs), the two resulting chromosomes will be red-normal 

 IkX, and white abnormal, WAb, which represent the 

 other (the cross-over) kinds of gametes of the F 1 female. 

 The ratio in which they are produced is the gametic 

 ratio and is a measure of the linkage. 



In the Fj males there is but one X chromosome, hence 

 there is no opportunity for interchange here between 

 the X chromosomes. The mate of the X chromosome is, 

 in the male, the Y chromosome. Other experiments have 

 shown that the Y chromosome carries no factors; hence 

 interchange seems precluded; and, so far, no loss of X 

 chromosome factors to the Y chromosome has ever been 

 observed. The X chromosome passes into the female- 



