mated to a red eyed male (top row). All the mature eggs of such a female
contain one white-producing X chromosome represented by the open bar in the
diagram. The red eyed male contains female-producing X-bearing sperm that
carry the factor for red eye color, and male-producing Y chromosomes. Any
egg fertilized by an X-bearing sperm will become a red eyed female because
the X chromosome that comes from the father carries the dominant factor for
red eye color. Any egg fertilized by a Y-bearing sperm will become a male
with white eyes because the only X chromosome that the male contains comes
from his mother and is white producing.

When these two F_1 flies are inbred (middle row) the following combinations
are expected. Half the eggs will contain each a white producing X
chromosome and half red producing. The female-producing sperms will each
contain a white X and the male-producing sperms will each contain an
indifferent Y chromosome. Chance meetings of egg and sperm will give the
four F_2 classes (bottom row). These consist of white eyed and red eyed
females and white eyed and red eyed males. The ratio here is 1:1 and not
three to one (3:1) as in other Mendelian cases. But Mendel's law of
segregation is not transgressed, as the preceding analysis has shown; for,
the chromosomes have followed strictly the course laid down on Mendel's
principle for the distribution of factors. The peculiar result in this case
is due to the fact that the F_1 male gets his single factor for eye color
from his mother only and it is linked to or contained in a body (the X
chromosome) that is involved in producing the females, while the mate of
this body--the Y chromosome--is indifferent with regard to these factors,
yet active as a mate to X in synapsis.

[Illustration: FIG. 60. Diagram of sex determination in type with XX female
and XO male (after Wilson).]

In man there are several characters that show exactly this same kind of
inheritance. Color blindness, or at least certain kinds of color blindness,
appear to follow the same scheme. A color blind father transmits through
his daughters his peculiarity to half of his grandsons, but to none of his
grand-daughters (fig. 38A). The result is the same as in the case of the
white eyed male of Drosophila. Color blind women are rather unusual, which
is expected from the method of inheritance of this character, but in the
few known cases where such color blind women have married normal husbands
th