THE DETERMINATION OF SEX 285 



maternal side, while the F x females are black because 

 their single s-chromosomes came from their black 

 father. 



6. NON-DISJUNCTION 



A striking confirmation of the chromosomal inter- 

 pretation of sex is furnished by the phenomenon of 

 non-disjunction discovered in 1913 by Bridges. In 

 attempting to explain certain unexpected ratios which 

 he obtained in a long series of breeding experiments 

 upon white-eyed Drosophilas, Bridges found that his 

 results would be more intelligible if what he termed 

 "non-disjunction" was assumed to occur. 



By non-disjunction is meant that both the #-chro- 

 mosomes instead of disjoining and going normally to 

 the two poles during the last maturation division, 

 remain attached to each other and pass together to 

 one pole leaving the other pole without any #-chromo- 

 some. In consequence, half the mature eggs should be 

 provided with two d?-chromosomes and half with none 

 at all. Cytological examination of these unusual flies 

 showed that this was what actually did sometimes 

 happen. 



The progeny of non-disjunctional white-eyed females, 

 as shown in Figure 90 taken from Sharp's "Introduc- 

 tion to Cytology", show a theoretical diversity of 

 characters which is borne out in the results of actual 

 breeding. Morgan sums the matter up when he says : 

 "An abnormal distribution of sex-chromosomes goes 

 hand in hand with an abnormal distribution of all sex- 

 linked factors." 



