the usual 3:1
Mendelian ratio. Yet, as will be shown later, the result is in entire
accord with Mendel's principle of segregation.

[Illustration: FIG. 36. Diagram illustrating a cross between a red eyed
male and white eyed female of the fruit fly (reciprocal cross of that shown
in Fig. 35).]

It has been shown by Sturtevant that in a wild species of Drosophila, viz.,
D. repleta, two varieties of individuals exist, in one of which the thorax
has large splotches and in the other type smaller splotches (fig. 37). The
factors that differentiate these varieties are sex linked.

[Illustration: FIG. 37. Two types of markings on thorax of Drosophila
repleta, both found "wild". They show sex linked inheritance.]

Certain types of color blindness (fig. 38) and certain other abnormal
conditions in man such as haemophilia, are transmitted as sex linked
characters.

[Illustration: FIG. 38, A. Diagram illustrating inheritance of color
blindness in man; the iris of the color-blind eye is here black.]

[Illustration: FIG. 38, B. Reciprocal of cross in Fig. 38 a.]

In domestic fowls sex linked inheritance has been found as the
characteristic method of transmission for at least as many as six
characters, but here the relation of the sexes is in a sense reversed. For
instance, if a black Langshan hen is crossed to a barred Plymouth Rock cock
(fig. 39), the offspring are all barred. If these are inbred half of the
daughters are black and half are barred; all of the sons are barred. The
grandmother has transmitted her color to half of her granddaughters but to
none of her grandsons.

[Illustration: FIG. 39. Sex-linked inheritance in domesticated birds shown
here in a cross between barred Plymouth Rock male and black Langshan
female.]

[Illustration: FIG. 40. Reciprocal of Fig. 39.]

In the reciprocal cross (fig. 40) black cock by barred hen, the daughters
are black and the sons barred--criss-cross inheritance. These inbred give
black hens and black cocks, barred hens and barred cocks.

There is a case comparable to this found in a wild species of moth, Abraxas
grossulariata. A wild variation of this type is lighter in color and is
known as A. lacticolor. When these two types are crossed they exhibit
exactly the same type of heredity as does the black-barred combination in
the domestic fowl. As shown in figure 41, lacticolor female bred to
grossulariata male gives grossulariata sons and daughters. These inbred
give grossularia