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, if 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 ('10) 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 neyer 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 Fi generation in either sex and in the 

 F 2 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 



