heterozygous male _grossulariata_ mated with _lacticolor_ female transmits
the character equally to both sexes: that is to say, the heredity is
completely sex-limited in the female but not at all in the male. This is
evidence that the female produces two kinds of eggs, one male producing
and the other female producing.

With regard to the ordinary form of colour-blindness, Bateson's first
explanation was that it was like the horns in the cross-bred sheep,
dominant in males, recessive in females. About 4 per cent. of males in
European countries are colour-blind, but less than 1/2 per cent. of
females. Affected males may transmit the defect to their sons but not to
their daughters: but daughters of affected persons transmit the defect
frequently to their sons. Bateson gives [Footnote: _Mendel's Principles of
Heredity_, 1909.] a scheme of the transmission, but corrects this in a
note stating that colour-blindness does not descend from father to son,
unless the defect was introduced by the normal sighted mother also, _i.e._
was carried by her as a recessive. The fact that unaffected males do not
transmit the defect shows, according to Bateson, that it is due to the
addition of a factor to the normal, not to omission of a factor.

According to later researches as quoted by Doncaster, colour-blindness is
due to the loss of some factor which is present in the normal individual.
The normal male is heterozygous for this normal factor. If we denote the
presence of the normal factor by _N_ and its absence or recessive by _n_,
then the male is _Nn_, while the female is homozygous or _NN_. But in
addition to this it is the male in this case which is heterozygous
for sex, and _n_ goes to the male-producing sperms, _N_ to the
female-producing. Thus in the mating of normal man with normal woman the
transmission is as follows:--