I04 MENDELISM chap, x 



carried out their experiments, the number of chromo- 

 somes is 8. Of these, two pairs are aHke and larger 

 than the others (Fig. 30), one pair is very small, the 

 remaining pair, the sex-chromosomes, being inter- 

 mediate in size. In the female these take the form 

 oi 2 X chromosomes, plain rod-like bodies. In the 

 male there is an X chromosome, similar to those 

 found in the female, and a Y chromosome of much 

 the same size, but differing from the X chromosome 



in being bent into a hook 

 at one end. All of the 

 eggs therefore contain 

 an X chromosome, but 

 of the sperms half con- 

 tain an X chromosome, 

 and half contain a Y 



The chromosomes of the two sexes in Z?r^j^. chromOSOmC. Whcuan 

 /A?V« magnified about 5000 times. (After • r ^'i* j l ir 



Bridges.). In the 9 the two X are the ^gg IS tertlllSed by a r 

 lowest pair in the figure ; in the S their ... . . 



place is taken by a rod-shaped Ji-chromo- SpCrm it glVCS HSC tO 

 some and a hook-shaped V chromosome. i xt • J 



a male. Now m order 

 to bring the chromosomes into relation with the 

 facts of sex-limited inheritance, we have to make 

 the assumption that the factors for sex - limited 

 characters are carried by any X chromosome, but 

 not by a y chromosome, nor, of course, by any other 

 of the pairs of chromosomes. In so far as this 

 group of characters is concerned the Y chromosome 

 behaves, as it were, like a dummy. Fig. 3 I shows 

 graphically how the results of the cross between the 

 white-eyed male and the red-eyed female may be 

 interpreted on these lines. Each of the X chromo- 

 somes of the female carries the factor for red, but 

 this factor is lacking in the X chromosome of the 



