are tortoise-shell as before, but the males yellow. The Mendelian
hypothesis which explains these results is that the male is always
heterozygous, or has only one colour factor whether yellow or black, and
transmits these colours only to his daughters, while the female has two
colour factors, either _BB_, _YY_, or _BY_. Thus the crosses are:--

YELLOW male x BLACK female
YO male BB female
| \/ |
| /\ |
YB female BO male
Tortoise-shell female BLACK male


BLACK male x YELLOW female
BO male YY female
| \/ |
| /\ |
BY female YO male
Tortoise-shell female YELLOW male

The sex must be determined therefore by the spermatozoa, as in the case of
colour-blindness, etc., in man, and the colour factor must always be in
the female-producing sperm.


SEXUAL DIMORPHISM

It is obvious from the above facts that however interesting and important
sex-linked heredity may be, it is not the same thing as the heredity of