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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.
They are all therefore red-eyed, but heterozygous that is, the red eye is due to one red-eye factor, not two. When the F1 are bred together, half the female gametes carry one X red chromosome, the other half one X white chromosome; half the male gametes carry one X red chromosome, the other half one Y white chromosome.
If such a gamete is fertilised by a normal gamete the organism developed from the zygote will be heterozygous, and segregation will take place in its gametes between the chromosome carrying the factor and the other without it, so that there will now be many gametes destitute of the factor in question.
F1 Red-eyed male Red-eyed female XR XW XR YW F2 Red-eyed male Red-eyed male Red-eyed female White-eyed female XR XR XW XR XR YW XW YW Homozygous. Heterozygous. Heterozygous. Homozygous. 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.
In the F1 generation the males were horned, the females hornless. Here, with regard to the horned character, both sexes were of the same genetic composition, i.e. heterozygous, or if we represent the possession of horns by H, and their absence by h, both sexes were Hh.
Or it might be somehow due to what Morgan and his colleagues have called crossing over in the segregation of heterozygous chromosomes, so that a part corresponding to a sinistral body is united with a part corresponding to a dextral head. My conclusion from the evidence is that any process of congenital development may in particular zygotes exhibit a mutation, a departure from the normal.
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.
On this last hypothesis the mutation here considered might be a heterozygous specimen, with the dextral condition dominant in the head and the sinistral in the body.
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
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.