White-eyed male Red-eyed female
XW XW x XR YW

F1 Red-eyed male White-eyed female
XW XR XW YW

F2 White-eyed male Red-eyed male White-eyed female Red-eyed female
XW XW XR XW XW YW XR YW
Homozygous. Heterozygous. Homozygous. Heterozygous.

It must be explained that according to this theory the normal male is
always heterozygous, because the _Y_ chromosome never carries any other
factor except that for sex; it is thus of no more importance than the
absence of an _X_ chromosome which occurs in those cases where the male
has one sex-chromosome and the female two. According to the researches of
von Winiwarter [Footnote: 'Spermatogenese humaine,' _Arch. de Biol._,
xxvii., 1912.] on spermatogenesis in man, the latter is actually the case
in the human species. This investigator found that there were 48
chromosomes in the female cell, 47 in the male; after the reduction
divisions the unfertilised ova had 24 chromosomes, half the spermatids 24
and half 23, so that sex is determined in man by the spermatozoon.

Morgan believes that the heredity of haemophilia (the constitutional
defect which prevents the spontaneous cessation of bleeding) follows the
same scheme, and also at least some forms of stationary night-blindness--
that is, the inability to see in twilight.

We may mention a few other in animals, referring the reader for a fuller
account to the works cited. One example in the barred character of the