8 MENDELIAN SEGREGATION 
that the X chromosome is attached to another 
chromosome and that there is no Y chromosome. 
In the earlier 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. 
Tue 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, P;), the offspring 
will have long wings (Fig. 3, F,). If these hybrid flies 
of the first generation (the first filial generation, or 
F,) are mated to each other, their offspring (or F.) 
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. 
