. 57. Scheme of sex determination in Drosophila type.
Each _mature_ egg contains one X, each mature sperm contains one X, or a Y
chromosome. Chance union of any egg with any sperm will give either XX
(female) or XY (male).]

[Illustration: FIG. 58. Cross between white eyed male of D. ampelophila and
red eyed female. The sex chromosomes are indicated by the rods. A black rod
indicates that the chromosome carries the factor for red; the open
chromosome the factor for white eye color.]

The first of these represents a cross between a white eyed male and a red
eyed female (fig. 58, top row). The X chromosome in the male is represented
by an open bar, the Y chromosome is bent. In the female the two X
chromosomes are black. Each egg of such a female will contain one "black" X
after the polar bodies have been thrown off. In the male there will be two
classes of sperm--the female-producing, carrying the (open) X, and the
male-producing, carrying the Y chromosome. Any egg fertilized by an X
bearing sperm will produce a female that will have red eyes because the X
(black) chromosome it gets from the mother carries the dominant factor for
red. Any egg fertilized by a Y-bearing sperm will produce a male that will
also have red eyes because he gets his (black) X chromosome from his
mother.

When, then, these two F_1 flies (second row) are inbred the following
combinations are expected. Each egg will contain a black X (red eye
producing) or a white X (white eye producing) after the polar bodies have
been extruded. The male will produce two kinds of sperms, of which the
female producing will contain a black X (red eye producing). Since any egg
may by chance be fertilized by any sperm there will result the four classes
of individuals shown on the bottom row of the diagram. All the females will
have red eyes, because irrespective of the two kinds of eggs involved all
the female-producing sperm carry a black X. Half of the males have red eyes
because half of the eggs have had each a red-producing X chromosome. The
other half of the males have white eyes, because the other half of the eggs
had each a white-producing X chromosome. Other evidence has shown that the
Y chromosome of the male is indifferent, so far as these Mendelian factors
are concerned.

[Illustration: FIG. 59. Cross between red eyed male and white eyed female;
reciprocal cross of Fig. 58.]

The reciprocal experiment is illustrated in figure 59. A white eyed female
is