N on- Disjunction in Drosophila. 41 



In the cytological examination one female was found with 10 

 chromosomes, i.e., XXYY. She was from a stock culture in 

 which half the females should be XXY and about half the males 

 XYY. Nearly half the eggs of such females are XY, and one-third 

 of the sperm of an XYY male are XY. Hence XXYY females 

 should frequently occur in this stock. 



The relations of non-disjunction to crossing over are still 

 more intricate and surprising in their results. In XXY females, in 

 cases in which the synapsis was XY (heterosynapsis) since the 

 X chromosome was in synapsis with Y, there could be no crossing 

 over with X. The XX eggs should therefore always be non- 

 crossovers, and this was shown to be true. Linkage experiments 

 show that there is about 33% of crossing over between eosin and 

 vermilion, i.e., if they entered the cross in separate X chromosomes 

 one-third of the eggs would carry neither factor or both in their 

 single X. But when an XXY female carried eosin in one of her 

 X chromosomes and vermilion in the other, the exceptional 

 daughters were always like their mother in that they still carried 

 eosin in one X and vermilion in the other. On the other hand, 

 in the XY and X eggs from an XXY female, crossing over took 

 -place in the ordinary way. 



Such XXY females have been obtained from three sources, (1) 

 XX egg + Y sperm, (2) XY egg + X sperm, (3) X egg + XY 

 sperm. In all cases the frequency of secondary exceptions is the 

 same, which means that the method of synapsis is the same 

 whether the two chromosomes came from the same or from 

 different parents. 



One must agree with Bridges that these results, which are 

 only summarized here, furnish a definite proof of the chromosome 

 theory of heredity. Exceptions which seemed at first to be 

 unconformable to any possible chromosome theory of heredity 

 have turned out to be a brilliant confirmation of the whole position. 

 In no other case have breeding and cytological work been combined 

 with more convincing results. The correctness of the point of 

 view involved has been independently demonstrated, first by 

 breeding experiments and then by cytological observation. 



There are certain interesting and significant differences 

 between the results of duplication of a chromosome in plants and 

 animals. As we have seen, individuals containing an extra Y 

 chromosome in the nuclei, or even an extra X as well (XXYY), 

 show no external peculiarity and are only distinguishable by the 



