chromosome and one _Y_ white chromosome. They are all therefore red-eyed,
but heterozygous--that is, the red eye is due to one red-eye factor, not
two. When the _F1_ are bred together, half the female gametes carry one
_X_ red chromosome, the other half one _X_ white chromosome; half the male
gametes carry one _X_ red chromosome, the other half one _Y_ white
chromosome. The fertilisations are therefore one _X_ red _X_ red, one _X_
red _X_ white, one _X_ red _Y_ white, and one _X_ white _Y_ white. These
last are the white-eyed males. The two different crosses are represented
diagrammatically below, the dark rod representing the _X_ red chromosome,
the clear rod the _X_ white chromosome, and the bent clear rod the _Y_
white chromosome.

According to Morgan, the heredity of colour-blindness in man is to be
explained exactly in the same way as that of white eye in _Drosophila_.
A colour-blind man married to a normal (homozygous) woman transmits the
peculiarity to half his grandsons and to none of his grand-daughters.
Colour-blind women are rare, but in the few cases known where such women
have married normal husbands the defect has appeared only in the sons, as
in the second of the diagrams below.

Parents Red-eyed male White-eyed female
XR XR x XW YW

F1 Red-eyed male Red-eyed female
XR XW XR YW

F2 Red-eyed male Red-eyed male Red-eyed female White-eyed female
XR XR XW XR XR YW XW YW
Homozygous. Heterozygous. Heterozygous. Homozygous.