138 
R. Ruggles Gates. 
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 
