8 MENDELIAN SEGREGATION 



that the X chromosome is attached to another 

 chromosome and that there is no Y chromosome. 

 In the earher papers on Drosophila this relation of 

 the chromosomes was assumed to be correct and the 

 female was represented as XX and the male as XO. 

 In Drosophila, then, there is a numerical corre- 

 spondence between the number of hereditary groups 

 and the number of the chromosomes. Moreover, the 

 size relations of the groups and of the chromosomes 

 correspond. The method of inheritance of the 

 factors carried by these chromosomes will now be 

 considered more in detail. 



The Inheritance of one Pair of Factors 



The inheritance of a single pair of characters may 

 be illustrated by the following examples from Droso- 

 phila, one from each of the four groups. 



The mutant stock called vestigial is so char- 

 acterized because it has only small vestiges of the 

 wings. If a fly with vestigial wings is mated to the 

 wild type with long wings (Fig. 3, Pi), the offspring 

 will have long wings (Fig. 3, Fi). If these hybrid flies 

 ^of the first generation (the first filial generation, or 

 Fi) are mated to each other, their offspring (or F2) 

 will be of two sorts: some will have long wings and 

 others will have vestigial wings. There will be three 

 times as many flies with long wings as flies with 

 vestigial wings. This is the Mendelian ratio of 

 3:1 that appears when a single pair of characters is 

 involved. 



