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 study of the chromosomes of the 
non-disjunctional females. He finds that such 
2 
4 X 
X 
Fic. 49.—Group of chromosomes of an XXY 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 group of chromosomes 
of the female of Drosophila ampelophila 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 
