7. T. H. MORGAN, op. cit., p. 306. 8. H. S. JENNINGS, op. cit., pp. 15-18. 9. Ibid., pp. 27-28. 10. R. PEARL, quoted by STOCKARD, see next reference, p. 218. 11. C. R. STOCKARD, J. Exper. Zoöl., May, 1918, pp. 221-222. 12. C. R. Stockard, Wistar Institute of Anatomy & Biology, Supplementary Bulletin No. 6, (1925), p. 21. 13. H. S. JENNINGS, op. cit., p. 48. 14. W. E. CASTLE, Heredity, pp. 35ff. CHAPTER XII PROBLEMS OF HUMAN HEREDITY Lord, I find the genealogy of my Saviour strangely 1. Roboam begat Abia; that is a bad father begat 2. Abia begat Asa; that is a bad father begat a 3. Asa begat Josaphat; that is a good father begat 4. Josaphat begat Joram; that is a good father begat I see, Lord, from hence, that my father's piety can- -THOMAS FULLER, Scripture Observations. No two human beings are exactly alike, not even identical twins. Our friends seldom make even momentary mistakes in identifying us though rarely have they consciously sought to know our physical details. These differences extend to all parts of the body including the brain and nervous systems. Some of them are due to environmental conditions; others to heredity. Yet, we are in the habit of assuming identity of qualities in our systems of law, ethics, and customs, though we make countless exceptions in actual life. What then is a normal man? Ask a friend this question and see if his definition does not apply to the ideal rather than to the normal. The point is that in our mental images of people we exclude as much as we include. We prefer this to that; we emphasize this and not that. Prejudice and sentiment confuse our judgments. Social tradition blinds our eyes to facts. When we enter the field of human heredity it is well to keep these habits in mind. To avoid (possibly to provoke) discussion let us define a normal man as one who can adjust himself to his social environment and earn an honest living. INHERITANCE OF PHYSICAL QUALITIES The evidence indicates that any inborn trait can be passed along, generation after generation. There is much less certainty as to unit characters in man than there was in the early days of Mendelian enthusiasm and practically all of these now recognized are undesirable abnormalities. There is no reason to doubt that many characters follow the Mendelian rules of inheritance, but Conklin's warning should be remembered : When we come to the development of more complex things such as temperament, feeble-mindedness, insanity, personality, we are dealing with the most complex phenomena in all the world-inconceivably more complex than any of the problems of astronomy, physics or chemistry. If eye color in the fruit fly is dependent upon a large number of inheritance factors, as Morgan and his pupils have shown to be the case, how much more probable is it that epilepsy, feeble-mindedness, genius and insanity are dependent upon a still larger number of inheritance factors, as well as upon an innumerable number of environmental causes? We may be sure that when the whole "alphabet of degeneracy from alcoholism to wanderlust" is attributed to the lack of a single hereditary factor, there has been a pitiful failure to recognize the complexity of the phenomena in question.1 It is true that there seem to be a few simple cases. Blue eyes act as recessives and brown eyes as dominants, as a rule. Fourteen hundred Danish children were examined by Winge with the following results: In the main the above results tally with the Mendelian scheme if the brown-eyed parents are carrying a gene for blue, i.e., if they themselves are hybrids in so far as eye color is concerned. Assuming that the eye colors were correctly determined, the twelve children with brown eyes coming from two blue-eyed parents do not conform. Dark hair also seems dominant to light; curly hair to straight. Such phenomena cause no special concern for they seem to have little importance. We see both blue-eyed and browneyed people in all walks of life. When, however, we discover eyes of such formation that vision is affected, or peculiar hands not adapted to the use of ordinary tools, we are more disturbed. Color blindness, especially in the common type, the inability to distinguish red and green, is in some professions a serious matter. Color-blind men are much more common than colorblind women. The children of a color-blind man, married to a normal woman, are normal, but if the girls of this generation marry, half of their sons will be color blind. A color-blind woman is found only when a color-blind man marries a woman whose father was color blind. If it be true that man is color blind when simplex, while the woman only when she is duplex, we should expect all the sons of a colorblind woman to have the trait. To this no exception is known. Bateson was able to find seventeen such cases, and all the sons were color blind. We have all seen human albinos who are characterized by an absence of pigment cells so that the skin looks chalky white while the eyes are red, due to the presence of blood. Their vision is usually very poor. This appears to be a recessive condition. On the other hand, the affliction known as hypotrichosis (hairlessness accompanied by loss of teeth), appears to be a dominant. FIG. 20. THE INHERITANCE OF HEMOPHILIA Even more serious is the sex-linked trait known as bleeding (hæmophilia). The smallest cut is likely to be dangerous to one having the trouble, as the clot does not coagulate. In the family charted above all the sons died in this fashion. The daughters do not have the trait. They will, however, transmit it to their sons but not to their daughters unless they chance to marry men who are also carriers. There is considerable variation in the shape of the human hand. Most of these forms have little significance but some of them are important as when the fingers all coalesce into a hook. The common variants are of three types: (1) Polydactylic, with an extra finger (extra toes are also common); (2) brachydactylic, with short, stumpy hands and fingers; and (3) syndactylic, with more or less of a web between the fingers. These conditions are said to be dominant and to be |