defect to one-half of their sons. From a glance at the diagram it is readily seen also that a color-blind female could result from the union of a color-blind man (see first row) and the daughter of a color-blind man (see third row). For half of the gametes of such a female would bear the defect as would also that half of the gametes of the male which carry _X_, hence the expectation would be that half of the daughters of such a union would be color-blind and half would be carriers of color-blindness; and that half of the sons would be color-blind and half normal. All the sons of a color-blind woman would be color-blind because she has only defective _X_-elements to pass on. The inheritance of various other conditions in man follows more or less accurately the same course as color-blindness. Among these may be mentioned: _hemophilia_, a serious condition in which the blood will not clot properly, thus rendering the affected individual constantly liable to severe or fatal hemorrhage; near-sightedness (_myopia_) in some cases; a degenerative disease of the spinal cord known as _multiple sclerosis_; progressive atrophy of the optic nerve (_neuritis optica_); Gower's _muscular atrophy_; some forms of _night-blindness_; in some cases _ichthyosis_, a peculiar scaly condition of the skin. In one of my own tabulations of a case of inheritance of "webbed" digits or _syndactyly_, a condition in which two or more fingers or toes are more or less united, a sex-linked inheritance is clearly indicated (Fig. 15), although from the pedigrees recorded by other investigators this condition usually appears in some of both the sons and daughters of an affected individual. [Illustration: FIG. 15 Chart showing the inheritance of a case of syndactyly after the manner of a sex-linked character. The affected individuals are represented in black; squares indicate males, circles females. The condition is seen to be inherited by males through unaffected females.] =The Occurrence of Sex-Linkage in Lower Forms Renders Experiments Possible.--=The course followed by such characters in man can be inferred only from the pedigrees we can obtain from family histories. Fortunately, however, such sex-linkage also occurs in lower animals and we are able therefore to verify and extend our observations by direct experiments in breeding. Several sex-linked characters have been found to exist in a small fruit-fly known as _Drosophila_. Extensive breeding experiments with this fly by Professor T. H. Morgan and his pupils have borne out remarkably the interpretation that the characters in question are really linked with a sex-determining factor. CHAPTER III MENDELISM