REVIEW OF BIOLOGY OF SEX-DETERMINATION 
13 
blind male would result in the production of 50 per cent color- 
blind females as shown in the diagram, figure 6. 
In animals in which the female is heterogametic (Lepidoptera 
and birds) sex-linked characteristics are likewise known to exist. 
In fact the first sex-linked characteristics were discovered in 
moths by Doncaster. In these cases it is the female which 
possesses the mechanism whereby the character in question can 
only be present once. Let us take, for example, barring, a 
White Eyed Female 
X X 
Red Eyed Male 
X Y 
Gametes and Combinations 
X X 
X X 
Red Eyed Females 
Gametes 
F* 
X Y 
White Eyed Males 
and Combinations 
( 1 ) 
XX 
Red Eyed 
Females 
ps 
( 2 ) ( 3 ) ( 4 ) 
XX X Y X Y 
White Eyed Red Eyed White Eyed 
Females Males Males 
Fig. 5. Diagram Showing Cross Reciprocal to that Shown in Figure 4 
dominant sex-linked trait in poultry. As shown in figure 7, 
the F2 generation results in three barred to one black, but the 
black individuals are all females. 
The reciprocal cross likewise shows that the character for 
barring follows the distribution of the Z-chromosome. As 
indicated in figure 8, the males are barred because they got 
a Z-chromosome from their maternal side; the F^ females are 
black because their lone Z-chromosome came from the paternal 
side. 
