196 DISTRIBUTION OF THE CHROMOSOMES 
late only adds an unknown and improbable assump- 
tion and leaves the situation less clear than before. 
The advantage of the chromosomal interpretation 
as applied to the sex chromosomes is nowhere better 
illustrated than in the history of a process called 
non-disjunction, which was discovered by Bridges. 
Furthermore this case, supported on the one hand 
by extensive and definite experimental breeding and 
on the other hand by cytological investigation, offers 
the most direct evidence yet obtained concerning 
the relations of particular characters and particular 
chromosomes, for in this case an abnormal distribu- 
tion of the sex chromosomes goes hand in hand with 
an identical abnormal distribution of all sex linked 
factors. It was found that females from a certain 
strain of white-eyed flies gave, on out-crossing, about 
5 per cent. of unexpected classes. For instance, 
one of the white females crossed to a red-eyed male 
(wild type) produced not only red-eyed daughters 
and white-eyed sons, as expected, but also a few 
white-eyed daughters and a corresponding number 
of red-eyed sons. The approximate percentage in 
which these classes appeared is as follows: 
Red 2 White @ White 9? Red 2 
47.5% 47.5% 2.5% 2.5% 
In general, therefore, there were 95 per cent. of 
expected forms and 5 per cent. of offspring that 
were apparently inconsistent with expectation on the 
chromosome theory. Closer inspection of these 
