■^c< 



?7 



104 MENDELISM chap, x 



carried out their experiments, the number of chromo- 

 somes is 8. Of these, two pairs are alike 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 Jf chromosome 



in being bent into a hook 



at one end. All of the 



eggs therefore contain 



•^^^ ^^ ^^__ an X chromosome, but 



• # J^!^^ of the sperms half con- 



^ ^ tain an X chromosome. 



Fig. 30. and half contain a Y 



The chromosomes ofthe two sexes in Zlroi^- chrOmOSOme. When an 

 /■//iVa magnified about 50QO limes. (After . r i_'i' i t Tr 



Bridges.) In the 9 the two X are the Cgg IS fertilised by a X 

 lowest pair in the figure ; in the i their ... . , 



place is taken by a rod-shaped ^chromo- Sperm it gIVeS riSe tO 

 some and a hooi^-shaped V chromosome. i XT • J 



a male. Now in 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 F 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 1 shows 

 graphically how the results of th? 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 



