152 DISTRIBUTION OF THE CHROMOSOMES 



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 study of the chromosomes of the 

 non-disjunctional females. He finds that such 



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

 tional '4ine." 



females contain an extra chromosome whose size 

 and position show that it is a supernumerary sex 

 chromosome. The normal group of chromosomes of 

 the female of Drosophila ampelophila is shown in 

 Fig. 2, 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 



