first exceptions noted concerned the principle of dominance. In experiments on the color of flowers and on various characters of poultry, it was often found that the hybrid did not show a pure dominant trait but was influenced more or less by the recessive trait even to the extent of a blend of the two characters. This fact is not inconsistent with Mendel's formula but serves to bring out some of the complexities of inheritance. Dominance is now regarded as merely a matter of degree. The nature of the characters crossed may be such that in combination one has little or no influence in determining the character of the hybrid; or their nature may be such that they are approximately equipotent in determining the character in the offspring. Dominance is usually noticeable when one character is positive and the other is negative; for example, a color is dominant over lack of color or albinism, though it is not always dominant over white. In other cases, where the character is not negative, but is still recessive, such as short hair recessive to long hair in animals, one explanation is that short hair contains a positive determinant which inhibits hair growth, and the presence of the determinant is dominant over its absence. However, it is unsafe to predict the dominance of a character without experiment, for sometimes unexpected results are obtained. Other exceptions concern the unit character theory and are even more difficult to work out. Many traits have been found in which the crossing of hybrids fails to produce the original characters but continues to produce some form of a blend. A familiar example in man is that of skin-color, and in the lower animals Castle obtained blends in his experiments on ear-lengths of rabbits and on the size of the various parts of the skeleton; and Bonhote found cases of continuous blends in his experiments on ducks. The explanation of this phenomenon is that certain body characters are not indivisible units but are composed of several factors which are the real units of inheritance. These factors remain distinct and segregate according to Mendel's law; but, inasmuch as many unit factors enter into a single character, these characters in future generations actually appear as varying combinations of the composite factors. These exceptions will be discussed in connection with the subject of human heredity. Inheritance of Physical Traits in Man. Inheritance in human beings, which is the most interesting part of the subject for sociologists, seems to be more complicated and to present more exceptions to Mendel's law than does inheritance in plants and lower animals. This may be apparent only, due partly to the difficulty of collecting material, for human inheritance is a slow process, experiment is impossible, and accurate data are difficult to obtain for every member of a family through several generations; not to mention the fact that occasionally, perhaps frequently, individuals have failed to mate and reproduce with due regard for the complete vindication of the Mendelian principles. A few characters in man do seem to behave as simple Mendelian units. The abnormality known as brachydactylism (two finger joints instead of three) seems to follow Mendel's law as a dominant character, though the case is only partially proved. None of the families reported either by Farrabee or by Drinkwater contained cases of marriage of two abnormals. Normal parents produce no abnormal offspring. Matings between normals and abnormals produce equal numbers of normals and abnormals, showing that the abnormals are hybrids, if the case is Mendelian in character. The partial, incomplete data gathered are in accordance with Mendelian expectations; but the best proof of segregation would come from the mating of two abnormals, which should produce both normal and abnormal offspring; and such matings have not been found. Night-blindness, haemophilia, and several other abnormalities, are also inherited as unit characters. Left-handedness is reported by Jordan, by Hurst, and by Ramaley as roughly fulfiling the requirements of a recessive character, though Hurst makes the provision that ambi-dexterity be counted as original left-handedness. Musical talent is thought by Hurst to be a recessive Mendelian trait, but this trait, as has been shown,' is a complex character, and therefore it is very doubtful if it could be made to fit into Mendel's formula as a simple recessive. Longevity is doubtless inherited, and Pearl presents some evidence to show that it follows Mendel's law, with short life recessive. 1 See Musical Quarterly, 6: 586. Human eye-color is supposed to follow strictly the Mendelian law, dark eye-color being dominant over blue or gray. Children of pure brown-eyed and blue-eyed parents will all be browneyed, though possibly the shade of brown will be lighter than in the parent. These children, however, will carry also the blue determinant; and they are technically called heterozygous, or simplex, for the character. A heterozygous, brown-eyed person mated with a blue-eyed person will have dark-eyed and blueeyed offspring theoretically in equal numbers. If both parents are heterozygous they will have blue-eyed as well as browneyed children in the average ratio of 1 to 3. If both parents are blue-eyed, all the children will be blue-eyed. If both parents are pure brown-eyed, - technically called homozygous for the character, - or duplex, all their offspring will be browneyed. This is the conclusion reached by Davenport, and is usually accepted as the rule of inheritance for eye-color; but it is only fair to say that Pearson, Boas, and Holmes have all reported cases inconsistent with Mendel's law; and it is not certain that eye-color is a good illustration of simple Mendelian inheritance in man. The inheritance of skin-color may be presented in some detail because it illustrates the principle of several independent factors combining to make up a character. Skin-color in the offspring of negro and white appears to be a blend; but Davenport has definitely shown that the color of the negro is made up of four distinct colors; and this permits five possible combinations, factor for black, one factor, two factors, three factors, or four factors. The offspring of two mulattoes show a wide range of variation from pure white to quite dark, because of the four possible factors, any one of which will darken the skin, and which may be inherited in varying combinations. The white children would be pure recessives and produce only white children; but where so many factors are involved the pure white is seldom found. In case of a single unit character, it has been shown that one in four will be a pure recessive. But if a larger number of factors are involved the original character will appear much less frequently. The following table gives the number of 1 Journal of Heredity, s: 556. 1 no a cases necessary, when several factors are present, to produce the original character according to the law of chance: 3 If 2 characters are involved 16 individuals for i recessive. 64 256 1,024 66 4,096 66 4 ( 6 Inasmuch as four factors are involved in skin color, only one in every 256 offspring of mulatto parents will be pure white. And even then such an individual would not be likely to be a Caucasian, for skin-color is not linked with curliness of hair, or with facial features, and the rare individual with pure white skin-color might still have other negro features. To ascertain the mode of human inheritance, a separate study has to be made of each character. Much remains to be done along this line, but the characters which have been studied, with their probable mode of inheritance, are given in the following table taken from Castle's Genetics and Eugenics. INHERITED CHARACTERS IN MAN 1. Blending General body size, stature, weight, skin-color, hair-form (in cross section, correlated with straightness, curliness, etc.) shape of head and proportions of its parts (features). 2. Mendelian Recessive Blonde or albino (probably multiple allelomorphs). Spotted with white. Uniformly colored. or scaly skin). hair Epidermolysis (excessive forma- Normal skin. tion of blisters). form). (eye blue). Hereditary cataract. Normal. Eyes Night blindness (When not sex- Normal. limited). Normal. Pigmentary degeneration of retina. Skeleton Brachydactyly (short digits and Normal. limbs). Normal. Normal. Normal. Normal. Alkaptonuria (urine black on oxidation). Normal. Hereditary feeble-minded Kidneys Nervous { Hortington's chorea. ness. 3. Mendelian and Sex-Linked (Appearing in males when simplex, but in females only when duplex.) Dominant Recessive Normal. Gower's muscular atrophy. Normal. Haemophilia (bleeding). Normal. Color blindness (inability to distinguish red from green). Normal. Night blindness (inability to see in faint light). 4. Probably Mendelian but Dominance Uncertain or Imperfect Defective hair and teeth or teeth alone, extra teeth, a double set of permanent teeth, hare-lip, cryptorchism and hypospadias (imperfectly developed male organs), tendency to produce twins (in some families determined by the father, in others by the mother), left-handedness, otosclerosis (hardness of hearing owing to thickened tympanum). 5. Subject to Heredity, but to what Extent or how Inherited Uncertain General mental ability, memory, temperament, musical ability, literary ability, artistic ability, mathematical ability, mechanical ability, congenital deafness, liability of abdominal hernia, cretinism (due to defective or diseased thyroids), defective heart, some forms of epilepsy and insanity, longevity.1 Inheritance of Mental Traits in Man. Turning from purely physical characters to the more important mental traits, the problem of inheritance becomes still more complicated and elusive. Genius, feeble-mindedness, epilepsy, and insanity, are 1 W. E. Castle, - Genetics and Eugenics (1920) p. 272. |