MENDELIAN SEGREGATION tt 
The four pairs of chromosomes of the female of 
Drosophila are shown in Fig. 2 (to the left). There 
are three pairs of large chromosomes and one pair of 
small chromosomes. One of the four pairs is the 
pair of sex or X chromosomes. In the male, Fig. 2 
(to the right), there are likewise three pairs of large 
chromosomes and a smaller pair. The two sex chro- 
mosomes in the male have been found to be dis- 
= J 
IN SY, 
Ae AY 
Fic. 2.— Diagram of female and of male group (duplex) of chromosomes 
of Drosophila melanogaster showing the four pairs of chromosomes. The 
hook on the Y chromosome is characteristic. The members of each pair 
are usually found together, as here. 
tinguishable from each other in shape. This distinc- 
tion was first observed in the odgonial figures of the 
XXY females that had arisen through non-dis- 
junction. Satisfactory figures of the spermatogonial 
groups were much more difficult to obtain, but these 
also showed that the Y was J-shaped and somewhat 
longer than the X. In length the chromosomes are 
in the ratio: X = 100: Y = 112: I] = 159: III = 
159: IV = 12. Stevens’ work had seemed to show 
