Barred male. Unbarred female.
Heterozygous. Homozygous.
B male B male X b female b male
B male b female b male b male
Barred female. Barred male.
Heterozygous. Heterozygous.]
This case is thus exactly similar to that of Abraxas grossulariata and A. lacticolor. The barred character like grossulariata is dominant, the unbarred recessive, and to explain the results it is necessary to assume that the female is not only heterozygous for the barred character, but also for sex, with the female sex-factor dominant. The recessive character in this case is linked to the female sex chromosome, or, as Bateson described it, the dominant character is repelled by the sex-factor. We may make a diagram of the kind given by Morgan if we use a rod of different shape for the female-producing sex-chromosome, and use the black rod for the dominant character:—
BARRED female x unbarred male
BX uY uX uX
| \/ |
| /\ |
BX uX uY uX
BARRED male unbarred female
Heterozygous Homozygous
BARRED male x unbarred female
BX BX uX uY
| \/ |
| /\ |
BX uX BX uY
BARRED male BARRED female
Heterozygous Heterozygous
Another case is that of tortoise-shell, i.e. black and yellow cats. The tortoise-shell with very rare exceptions is female, the corresponding male being yellow, without any black colour. Doncaster found that a yellow male mated to a black female produced black male offspring and tortoise-shell females. When a black male is mated to a yellow female, the female kittens 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