T. H. Morgan [Footnote: _A Critique of the Theory of Evolution._] has
observed a number of cases of sex-linked inheritance in the mutations
which occurred in his cultures of _Drosophila_. The eye of the wild
original fly is red, one of the mutants has a white eye, _i.e._ the red
colour and its factor are absent. When a white-eyed male is mated to a
red-eyed female all the offspring have red eyes. If these are bred _inter
se_, there are, as in ordinary Mendelian cases, three red-eyed to one
white-eyed in the _F2_ generation, but white eyes occur only in the males,
in other wards half the males are white-eyed. On the other hand, when a
white-eyed _female_ is mated to a red-eyed male all the daughters have red
eyes, and all the sons white eyes. This has been termed crisscross
inheritance. If these are bred together the result in _F2_ is equal
numbers of red-eyed and white-eyed females, and equal numbers of red-eyed
and white-eyed males. The ration of dominant to recessive is 2 to 2
instead of the usual Mendelian ration of 3 to 1.

According to Morgan the interpretation is as follows: In the nucleus
of the female gametocytes there are two _X_ chromosomes related to sex,
in those of the male there is one _X_ chromosome and one _Y_ chromosome
of slightly different shape. The factor for red eye occurs in the
sex-chromosomes, that is to say, according to this theory, the
sex-chromosome does not merely determine sex but carries other factors
as well, and this fact is the explanation of sex-linked inheritance. The
factor for red eye then is present in both _X_ chromosomes of the wild
female, absent from both _X_ and _Y_ chromosomes of the white-eyed male.
The gametes of the female each carry one _X_ red chromosome, of those of
the male half carry an _X_ white chromosome, and half the _Y_ white
chromosome. The fertilised female ova therefore carry an _X_ red
chromosome + an _X_ white chromosome, the male producing ova one _X_ red