6 SEX-LINKED INHERITANCE IN DROSOPHILA. 



Such cases of linkage have been described in several forms, but no- 

 where on so extensive a scale as in the pomace fly, Drosophila arnpelo- 

 phila. Here, over a hundred characters that have been investigated 

 as to their linkage relations are found to fall into four groups, the 

 members of each group being linked, in the sense that they tend to be 

 transmitted to the gametes in the same combinations in which they 

 entered from the parents. The members of each group give free 

 assortment with the members of any of the other three groups. A 

 most significant fact in regard to the linkage shown by the Drosophila 

 mutants is that the number of linked groups corresponds to the number of 

 pairs of the chromosomes. If the gens for the MendeHan characters 

 are carried by the chromosomes we should expect to find demonstrated 

 in Drosophila that there are as many groups of characters that are 

 inherited together as there are pairs of chromosomes, provided the 

 chromosomes retain their individuality. The evidence that the 

 chromosomes are structural elements of the cell that perpetuate them- 

 selves at every division has continually grown stronger. That factors 

 have the same distribution as the chromosomes is clearly seen in the 

 case of sex-linked characters, where it can be shown that any character 

 of this type appears in those individuals which from the known distribu- 

 tion of the X chromosomes must also contain the chromosome in ques- 

 tion. For example, in Drosophila, as in many other insects, there are 

 two X chromosomes in the cells of the female and one X chromosome 

 in the cells of the male. There is in the male, in addition to the X, also 

 a Y chromosome, which acts as its mate in synapsis and reduction. 

 After reduction each egg carries an X chromosome. In the male there 

 are two classes of sperm, one carrying the X chromosome and the other 

 carrying the Y chromosome. Any egg fertilized by an X sperm pro- 

 duces a female; any egg fertilized by a Y sperm produces a male. 

 The scheme of inheritance is as follows. 



The sons get their single X chromosome from their mother, and 

 should therefore show any character whose gen is carried by such a 

 chromosome. In sex-linked inheritance all sons show the characters 

 of their mother. ' A male transmits his sex-linked character to his 

 daughters, who show it if dominant and conceal it if recessive. But 

 any daughter will transmit such a character, whether dominant or 

 recessive, to half of her sons. The path of transmission of the gen 

 is the same as the path followed by the X chromosome, received here 



