DISTRIBUTION OF THE CHROMOSOMES 199 



The results bear out to a remarkable degree the 

 hypothesis that they are due to a non-disjunction 

 of the sex chromosomes caused by the presence of a 

 Y chromosome in the females. 



The hypothesis is capable of verification and 

 Bridges has made a stud}' of the chromosomes of the 

 non-disjunctional females. He finds that such 



Fig. 49. — Group of chromosomes of an XX\ female of a non-disjunc- 

 tional "line." 



females contain an extra chromosome whose size, 

 shape and position show it to be a supernumerary 

 Y-chromosome. The normal grouj) of chromosomes 

 of the female of Drosophila ampelopliila is shown 

 in Fig. 12, and a group from a non-disjunction 

 female in Fig. 49. They differ by one chromosome, 

 namely, the extra Y. 



One additional fact must be mentioned. If an 

 XXY female should be fertilized by an XYY male 



