84 PHYSICAL BASIS OF HEREDITY 



utilized in the preceding cases. Three, four, five, or, 

 theoretically, any number of characters may show this 

 relation to each other. Thus there is a stock of Droso- 

 phila with five linked mutant characters, namely, black, 

 purple, curved, plexus, speck. In a back-cross, like the 

 one above, all the mutant characters, if they went in 

 together, will come out together in half of the second 

 generation (back-cross) flies, and their wild type allelo- 

 morphic characters in the other half. 



There is another way in which linkage may be very 

 simply illustrated. There are certain characters, called 



Diploid Nuclei XX 



Gametes X X Y 



Fertilization 



Zygotes 



Fig. 35. — Scheme showing the inheritance of the sex-chromosome in Drosophila. 



sex-linked characters, because their factors follow the sex- 

 chromosomes, or may be said to be carried by them or to 

 be in them. Now in Drosophila, the female has two X- 

 chromosomes (Fig. 35), the male one X (and a Y). After 

 reduction the eggs have each one X chromosome. Any 

 Such ^gg fertilized by a Z-bearing sperm will produce a 

 male {XY), as shown in the scheme below. The single X- 

 chromosome that this male gets is therefore from his 

 mother. If her X-chromosome carried sex-linked factors, 

 these should be present in the son. Such, in fact, is the case. 

 For example, a female Drosophila with yellow wings and 

 white eyes mated to a wild-type male will produce wild- 

 type females, and yellow white-eyed sons (like the 



