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 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- 



9 



d 



a. /,«, 



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

 of Drosophila mclauogaster showing the four pairs of chromosomes. The 

 hook on the Y chrom<jsome 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 oogonial figures of the 

 XXY females that had arisen through non-dis- 

 junction. Satisfactory figures of the spermatogonia! 

 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: II = 159: III = 

 159: IV = 12. Stevens' work had seemed to show 



