MENDELIAN SEGREGATION 7 



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 is the pair of sex 

 chromosomes (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 are here represented as differ- 



FIG. 2. Diagram of female and of male group (duplex) of chromosomes 

 of Drosophila ampelophila showing the four pairs of chromosomes. The 

 hook on the Y chromosome is a convention. The members of each pair 

 are usually found together, as here. 



ing from each other in shape. In the diagrams 

 the Y chromosome is represented as hook shaped, 

 but this is intended only as a convention. It is true 

 that in the case of non-disjunction where the Y 

 chromosome has been transferred to the female it 

 has this hook shape, but as yet it has not been pos- 

 sible to identify the Y chromosome as hook shaped 

 in the male. Stevens' work had seemed to show 



