DISTRIBUTION OF THE CHROMOSOMES 149 



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 9 White cf White 9 Red cf 



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 



