chromosome. In the next generation color blindness can not appear since each fertilized egg contains the factor for color perception. In the second generation, however, the theory demands that one half of the males should be color blind. In man these conditions can not always be verified numerically since the number of children is too small to yield the conditions to be expected according to the calculus of probability. T. H. Morgan has found in a fly (Drosophila) a number of similar sex-limited characters, which behave like color blindness, e. g., lack of pigment in the eyes. These flies have normally red eyes. Morgan has observed a mutation with white eyes, which occurs in the male. When he crossed a white-eyed with a red-eyed female all flies of the first generation were red-eyed; since all flies had the factor for pigment in their sex-cells; in the second generation all females and exactly one half of the males had red eyes, the other half of the males, however, white eyes, as the theory demands.
From these and numerous similar breeding experiments of Correns, Doncaster, and especially of Morgan, we may conclude with certainty that the sex-chromosomes are the bearers of those hereditary characters which appear preeminently in one sex. We say preeminently since theoretically we can predict cases in which color blindness or white eyes must appear also in the female. Breeding experiments have shown that this theoretical prediction is justified. The riddle of Mendel's law of segregation finds its solution by these experiments and incidentally also the problem of the determination of sex which is only a special case of the law of segregation, as Mendel already intimated.
The main task which is left here for science to accomplish is the determination of the chemical substances in the chromosomes which are responsible for the hereditary transmission of a quality, and the determination of the mechanism by which these substances give rise to the hereditary character. Here the ground has already been broken. It is known that for the formation of a certain black pigment the cooperation of a substance—tyrosin—and of a ferment of oxidation—tyrosinase—is required. The hereditary transmission of the black color through the male animal must occur by substances carried in the chromosome which determine the formation of tyrosin or tyrosinase or of both. We may, therefore, say that the solution of the riddle of heredity has succeeded to the extent that all further development will take place purely in cytological and physico-chemical terms.
While until twelve years ago the field of heredity was the stamping ground for the rhetorician and metaphysician it is to-day perhaps the most exact and rationalistic part of biology, where facts can not only be predicted qualitatively, but also quantitatively.