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