TYPES OF MENDELIAN HEREDITY 29 
the heterozygous condition; it is not possible to 
produce a pure breeding race of Andalusians, for the 
combination that produced an Andalusian falls apart 
in the germ cells of the Andalusian birds. The bird is 
blue because the pigment is not spread evenly over 
the feather but is restricted to small but black specks. 
Fig. 15.—Normal (a, a’) and bar eye (8, b’) of Drosophila; shown in 
side view, and as seen from above. 
The Andalusian blue is a mosaic of black and white, 
and not at all a dilute black. 
A good example of an intermediate hybrid is found 
when the mutant fly with bar eye (Fig. 15) is bred to a 
wild fly. The daughters have bar eyes that are not 
as narrow as those of the pure bar stock. The range 
of variation is great, however, for some of the hybrids 
have eyes that are nearly as round as the normal, and 
