52 THE THEORY OF THE GENE 



an eyeless fly (from stock) the expectation is that there 

 will be five wild-type flies to one eyeless (Fig. 33, lower 

 half) instead of equality as in the ordinary case when a 

 heterozygous individual is back-crossed to its recessive. 

 The diagram (Fig. 33) shows the recombinations of 

 germ-cells that are expected to give rise to the 5 to 1 

 ratio. The actual number of eyeless obtained approxi- 

 mates expectation. 



These and other experiments of the same kind show 

 that the genetic results check up at every point with the 

 known history of chromosome-IV. No one familiar with 

 the evidence can doubt for a moment that there is some- 

 thing in this chromosome that is responsible for the 

 observed results. 



There is also evidence that the sex-chromosomes are 

 the bearers of certain genes. In Drosophila there are as 

 many as 200 characters whose inheritance is said to be 

 sex-linked. This term means only that they are carried 

 by the sex-chromosomes. It does not mean that the 

 characters are confined to one or the other sex. Owing to 

 the differential pair of sex-chromosomes in the male, the 

 X and the Y, the inheritance of characters whose gene 

 lies in the X-chromosomes is somewhat different from 

 that of anv of the other characters. There is evidence 

 that the Y-chromosome does not contain in Drosophila 

 any genes that conceal the recessives in the X. It may, 

 therefore, be ignored except in so far as it acts as the 

 mate of the X in the male at the reduction division of 

 the sperm-cells. The mode of inheritance of linked charac- 

 ters of Drosophila has already been given in Chapter I 

 (Figs. 11, 12, 13, 14). The mode of transmission of the 

 sex-chromosome is given in Fig. 38. An examination of 

 the latter shows that these characters follow the known 

 distribution of the chromosome. 



Occasionally the sex-chromosomes "go wrong," and 



