24 



SEX-DETERMINATION 



appearance. Bridges showed that these exceptional indi- 

 viduals could be explained if it were assumed that the 

 white-eyed mother was not (wX)(wX) but (wX)(wX)Y 

 in respect of sex-chromosome constitution, if a definite 

 abnormality in the distribution of the sex-chromosomes had 

 occurred during the maturation of the egg from which she 

 sprang. 



Normally, of the two X-chromosomes in the unripe egg^ 

 one during the reduction division passes into the polar body. 

 If exceptionally these X's did not so disjoin, both remaining 

 in the egg, or if both passed into the polar body, then two 

 further kinds of mature ova would result, one with two X's, 

 each in this case carrying the gene for white-eye (wX), and 

 the other without any X. The spermatozoa of the red-eyed 

 male are of two kinds, one with an X carrying the dominant 

 red-eye gene, the other with a Y. Fertilization of these two 

 exceptional kinds of egg would then yield four combina- 

 tions, so: 



Eggs abnormal in respect of 

 sex-chromosome constitution 



Spermatozoa 



because she received both of her X's from her mother. 



An exceptional red-eyed male. He received his X from his father 



and not from his mother. 



Does not appear. 



The mating of a normal red-eyed male to a non-disjunc- 

 tional white-eyed female (2 above) will yield the following 

 results: 



