REPRODUCTION AND HEREDITY 129 



Male Rw Female ww 



?(^ ?? 



the two kinds of offspring will be represented by 

 Males WW Females Rw 



and consideration of possibilities convinces us that the sex 

 and eye-colour of the progeny depends on the linkage of 

 the factor for red eyes with that for femininity in half the 

 ripe sperms of the hybrid males. This can only mean that 

 those two factors reside in the same chromosome, which at 

 a reducing division passes into one or other of a pair of 

 sperm-nuclei. 



Study of chromosome behaviour in the cells of Droso- 

 phila is comparatively simple because there are only four 

 pairs of those bodies — one pair minute and round, two 

 pairs bow-shaped and clubbed at their extremities, and a 

 pair of which in the female both members are straight and 

 rod- like, while in the male one is straight and the other 

 sharply bent towards one end. The members of this last 

 pair are the sex- chromosomes, the bent member of the 

 male pair being visibly the y chromosome ; this carries the 

 factors which determine the male sex and also that which 

 brings about absence of pigment in the eyes. The factor 

 for redness of eyes, if borne in a male Drosophila, is linked 

 with that for femaleness in the x chromosome. More than 

 a hundred such sex-linked characters have been studied by 

 Morgan and his colleagues in their breeding experiments 

 with Drosophila, and the way in which these are grouped 

 in inheritance correspond with the number of chromosomes, 

 four pairs, present in the zygote nuclei. 



Mention must be made of a few exceptional results 

 shown by these experiments which, when analysed, are 

 found to throw further light on the part played by the 

 chromosomes in heredity. In the families resulting from 

 the union of hybrid red-eyed male Drosophila with pure 

 white-eyed females, the offspring are not always all white- 



