Many hypotheses have been advanced to explain the inheritance of blood types, but none of them could account theoretically for the actual facts. My own hypothesis of linkage alone can perfectly explain the actual facts of inheritance both quantitatively and qualitatively. I observed from my own experience and statistical material that ab and AB gametes can be formed by crossing over from the AB type and I have been able to prove the validity of my own hypothesis theoretically and actually. There has been much controversy as to the kind of children that should be born of parents of the AB type, but I have been able to show that children of 0, A, B and AB types can be born of parents of the AB type. The rate of the appearance of the so-called “questionable children” born of parents of the AB type has been considered as irregular, and sometimes even their appearance has been doubted. Such a phenomenon, however, is not to be wondered at. When my hypotheses are accepted, it can be easily explained by the existence of the phenomenon of linkage, the numerical ratio of spermatozoa and ova, the conditions under which they are ejected, and the fecundity. When one refers to the literature concerned, it seems that the crossing over takes place comparatively frequently in the genital cells of the maternal body. But minute observation leads to the opinion that this phenomenon also depends upon the conditions under which spermatozoa and ova are ejected. The settlement of the question of the inheritance of blood types is of service in anthropological observation, and establishes a standard for the medico-legal judgment of parentship. Moreover the existence of the phenomenon of linkage, to which no great attention has hitherto been paid in the problem of human heredity, has been demonstrated. It is to be supposed that the study of human heredity will develop in this direction on the basis of the present demonstration.