270 RECENT CYTOLOGY 



coupling. He supposes that the factors for all the 

 characters of a species which show gametic coupling 

 with one another are borne by the same chromosome. 

 In the Fruit -fly (Drosophila) there is rather striking 

 evidence that this is the case, for there are four groups 

 of coupled characters, which may be called A, B, C, D. 

 Any character in one group is more or less closely 

 coupled with any other character in the same group, 

 but quite independent of characters in the other 

 groups. Groups A, B, and C each include a number 

 of characters, while group D is very small, having only 

 two or three characters which are known at present. 

 Now, in the germ-cell of Drosophila there are four 

 chromosomes, three large and one very small, and the 

 inference is therefore made that each chromosome 

 corresponds with a group of coupled characters. 

 Further, one chromosome is an ' idiochromosome,' or 

 sex-chromosome (see the pages immediately following), 

 and all the characters of group A are ' sex-limited ' in 

 transmission i.e., they are transmitted by the male 

 only to those offspring which also receive the sex- 

 chromosome. The correspondence between the groups 

 of coupled characters and of the chromosomes in 

 Drosophila is thus so close as to make the hypothesis 

 very probable that the chromosomes actually are the 

 bearers of Mendelian characters.* 



* For further evidence of a different kind that these char- 

 acters are borne by chromosomes in Drosophila, the reader is 

 referred to C. B. Bridges' paper, 'Non-Disjunction as Proof of 

 the Chromosome Theory of Heredity,' in Genetics^ vol. i., No. i, 

 January, 1916, or to Morgan's ' Physical Basis of Heredity,' 1919. 



