CHAPTER XXXI 
SEX-LINKED AND OTHER KINDS OF LINKED INHERIT- 
ANCE IN DROSOPHILA AND OTHER SPECIES! 
WILLIAM E, CASTLE 
All the facts of sex-linked inheritance in Drosophila harmonize 
with Morgan’s hypothesis that the genes of sex-linked characters lie 
in a common cell structure (X-chromosome) which is duplex in females, 
simplex in males. Accordingly, in a race which breeds true for a 
sex-linked character, that character may be transmitted by every egg, 
but by only dalf the sperms, namely by such as possessan X-chromosome 
and by virtue of that fact determine as female all zygotes into which 
they enter. To male zygotes the sperm will not transmit sex-linked 
characters. This hypothesis is supported by some curious facts already 
alluded to but deserving of fuller consideration in this connection, viz., 
facts observed in reciprocal crosses involving a sex-linked character, 
as for example white-eye in Drosophila. 
TABLE I 
RECIPROCAL CROSSES OF WHITE-EYED WITH RED-EYED DROSOPHILA 
Male Female Male Female 
P White xX Red Red x White 
F, Red Red White Red 
F, 1 Red: 1 White Red 1 Red: 1 White 1 Red: 1 White 
It has already been stated that a white-eyed male Drosophila 
crossed with normal females has only normal children of both sexes, 
while the white-eyed grandchildren are all of the male sex. In the 
reciprocal cross, between a white-eyed female and a normal male all 
the daughters are normal, but the sons are white-eyed, and among the 
grandchildren white-eyed individuals occur in both sexes. Diagrams 
will best explain these facts on the basis of Morgan’s hypothesis. 
(See Figs. 87 and 88 and Table I.) 
To state the foregoing facts in another way, it will be observed that 
the recessive sex-linked character in Drosophila, when introduced in a 
cross by the male parent, disappears entirely in F, and reappears in F, 
From W. E. Castle, Genetics and Eugenics (copyright 1920). Used by 
special permission of the publishers, The Harvard University Press. 
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