6o 



GENETICS 



flies was discovered by L. V. Morgan in which the X- 

 chromosomes are stuck together, so that they almost never 

 separate. In this race germ cells are practically always 

 formed in the way just described. So it has been possible to 

 study the results thoroughly, since thousands of offspring 



A 



B 



C 



D 



Parents 



Fertilized Eggs 



Figure 20. Diagram to illustrate the results of the failure of the 

 two X's to separate, in the production of germ cells by the female. 

 A, Normal formation of germ cells in male, giving two sets of germ 

 cells, one containing Y, the other X. B, Non-disjunction in the fe- 

 male ; the two X's fail to separate, giving two sets of germ cells, one 

 containing two X's, the other with no X. C, female containing 

 XXY. D, male, containing X but no Y. 



have been obtained. These results are of great importance 

 and interest. 



From such individuals we have then two kinds of ova, 

 one AXX, with two X's; one AO, with no X (figure 20, B). 

 What will happen when these unite with the usual two kinds 

 of sperm, AX and AY? 



When the ovum AXX unites with a sperm AY, we get 

 offspring AAXXY (figure 20, B), containing two X's and 



