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- 

 pMla 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 

 Fertilization 

 Zygotes 



FiQ. 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 egg 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 wiU produce wild- 

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



