nn (male) x NN (female)
Gametes n (male) + n (female) x N + N
n (male) + N N (female) + N
| |
nN (male) nN (female)
When a normal male is mated with a heterozygous nN female we get
nN (male) x nN (female)
Gametes n (male) + N (female) x n + N
______________________|______________________
| | | |
nn (male) nN (male) nN (female) NN (female)
that is, half the sons are normal and half colour-blind, while half the females are homozygous and normal, and the other half heterozygous and normal.
T. H. Morgan [Footnote: A Critique of the Theory of Evolution.] has observed a number of cases of sex-linked inheritance in the mutations which occurred in his cultures of Drosophila. The eye of the wild original fly is red, one of the mutants has a white eye, i.e. the red colour and its factor are absent. When a white-eyed male is mated to a red-eyed female all the offspring have red eyes. If these are bred inter se, there are, as in ordinary Mendelian cases, three red-eyed to one white-eyed in the F2 generation, but white eyes occur only in the males, in other wards half the males are white-eyed. On the other hand, when a white-eyed female is mated to a red-eyed male all the daughters have red eyes, and all the sons white eyes. This has been termed crisscross inheritance. If these are bred together the result in F2 is equal numbers of red-eyed and white-eyed females, and equal numbers of red-eyed and white-eyed males. The ration of dominant to recessive is 2 to 2 instead of the usual Mendelian ration of 3 to 1.
According to Morgan the interpretation is as follows: In the nucleus of the female gametocytes there are two X chromosomes related to sex, in those of the male there is one X chromosome and one Y chromosome of slightly different shape. The factor for red eye occurs in the sex-chromosomes, that is to say, according to this theory, the sex-chromosome does not merely determine sex but carries other factors as well, and this fact is the explanation of sex-linked inheritance. The factor for red eye then is present in both X chromosomes of the wild female, absent from both X and Y chromosomes of the white-eyed male. The gametes of the female each carry one X red chromosome, of those of the male half carry an X white chromosome, and half the Y white chromosome. The fertilised female ova therefore carry an X red chromosome + an X white chromosome, the male producing ova one X red chromosome and one Y white chromosome. 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. The fertilisations are therefore one X red X red, one X red X white, one X red Y white, and one X white Y white. These last are the white-eyed males. The two different crosses are represented diagrammatically below, the dark rod representing the X red chromosome, the clear rod the X white chromosome, and the bent clear rod the Y white chromosome.
According to Morgan, the heredity of colour-blindness in man is to be explained exactly in the same way as that of white eye in Drosophila. A colour-blind man married to a normal (homozygous) woman transmits the peculiarity to half his grandsons and to none of his grand-daughters. Colour-blind women are rare, but in the few cases known where such women have married normal husbands the defect has appeared only in the sons, as in the second of the diagrams below.
Parents Red-eyed male White-eyed female
XR XR x XW YW
F1 Red-eyed male Red-eyed female
XR XW XR YW