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STUDIES ON CHROMOSOMES 95 
. tenso; but I wish to indicate some of the possibilities that have 
been opened by the cytological results, even at the risk of offering 
what may be regarded as too speculative a treatment of the matter. 
It is obvious that any recessive mutation should exhibit sex-limited 
heredity when crossed with the normal or dominant form, of it be due 
to a factor contained in (or omitted from) the X-element. For in- 
stance, in the remarkable Drosophila mutants discovered by 
Morgan (710) the experimental data establish the fact that white 
eye-color (which seems to follow the same type of heredity as 
color-blindness in man) is linked with a sex-determining factor in 
such a way that when the white-eyed male is crossed with the 
normal red-eyed female, the former character is never transmitted 
from father to son, but through the daughters to some of the 
grandsons (theoretically to 50 per cent), though the daughters are 
not themselves white-eyed; that is, after such an initial cross, white 
eyes fail to appear in the F, generation in either sex and in the 
F, generation appear only in some of the males. As Morgan 
points out, this follows as a matter of course if the factor for white 
eye be identical with, or linked with, a sex-determining factor in 
respect to which the male is heterozygous or simplex, the female 
homozygous or duplex. The X-element exactly corresponds in 
mode of distribution to such a sex-determining factor; for this 
chromosome, too, is simplex in the male, duplex in the female 
and its introduction into the egg by the spermatozoon produces 
the female condition, its absence the male. This chromosome 
therefore, as I have shown (’06), is never transmitted from father 
to son, but always from father to daughter. Conversely, the 
male zygote always receives this chromosome from the mother. 
So precise is the correspondence of all this with the course of sex- 
limited heredity of this type that it is difficult to resist the con- 
clusion that we have before us the actual mechanism of such 
heredity—in other words, that some factor essential for sex is 
associated in the X-element with one that is responsible for the 
sex-limited character. 
This will be made clearer by the accompanying diagram (fig. 
8) where the X-element assumed to be responsible for a recessive 
sex-limited character is underscored (X). This character may 
