xi v] NON-DISJUNCTION 233 



tution XXT ', will be a female, since two ^-chromosomes 

 are present, but will be white-eyed because no red-bearing 

 ^f -chromosome has been introduced from the male parent. 

 And if both Jf-chromosomes are extruded, and the egg is 

 fertilised by an ^-bearing spermatozoon, the zygote, of 



FIG. 29. Chromosome groups of Drosophila ampelophila (from original 

 drawings by C. B. BRIDGES). 



a. Normal oogonial group; the two lowest chromosomes are * X.' 

 b, c. Normal spermatogonial groups. The bent chromosome (left bottom) 

 is T,' the straighter rod on the right of it, 'X* 

 Some of the chromosomes are beginning to split longitudinally. 

 In b "the extra granule is probably of no significance" (BRIDGES). 



d, e, J. Oogonial groups of XXT females. 

 g. Oogonial group of an XXTY female. 



constitution X , will be male, but will be red-eyed because 

 the single ^-chromosome is derived from the red-eyed 

 father. Such "primary non-disjunction" of the sex-chro- 

 mosomes is very rare, but when it has once happened it leads 

 to the production of females of the constitution XXT, and 



