HEREDITY IN MAN There is now no doubt that the laws of heredity are the same in man and in other animals, but their application is very different. In the case of domestic animals a new type is often bred from extensively. There is thus a fair probability that two piebald cats will owe their origin to a single common ancestor, and that their condition will therefore be due to the same gene or genes. With two piebald men not known to be related the opposite is probably true. Again in-breeding and in consequence homozy gosity are far commoner in most other organisms than in man. It is convenient to divide human heritable characters into well marked characters exhibiting Mendelian inheritance, other physi cal characters, and other psychological characters.
Dominant Abnormalities.—Many well-marked hereditary abnormalities run in families. A dominant character is handed down regardless of sex, only by an affected individual, and to about half of his or her children. Among anomalies of the eyes alone, such characters include myopia, two or more types of presenile cataract, retinitis pigmentosa (in certain families), acute glaucoma, and night-blindness (in some families). A pedigree of the latter includes 2,116 individuals, of whom 135 were night-blind, all descended from Jean Nougaret, who was born near Montpellier in 1635. Other dominant characters include several ear and skin diseases, diabetes insipidus and other metabolic diseases. In some cases the inheritance is apparently somewhat irregular. Thus acholuric jaundice is strongly hereditary, but occasionally trans mitted through unaffected individuals. The latter however are found to have abnormally fragile red blood corpuscles, and it is this character which is actually inherited, though a person dis playing it may actually escape the usual consequence of jaundice. In addition a number of malformations are inherited as dominants, including brachydactyly (absence of one bone in each finger) and lobster-claw (absence of all digits except sometimes the thumb and little finger). Minor anom alies inherited in this way may prove very serious in the homo zygous form. Thus a marriage between cousins with a hereditary shortening of one finger produced a child without hands or feet.
The social importance of dom inant abnormalities, which are inherited only from affected per sons may be gauged by the fact that over io% of blindness in Germany is due to hereditary cataract alone. On the other hand many of the above com plaints can also be produced by environmental influences.
Sex-linked Recessive Abnormalities.—Another great group of abnormalities are sex-linked recessives. They are very much commoner in males than in females, and are not passed on by affected males to their children, though their daughters, who receive an abnormal X-chomosome from them, hand the character on to half their sons. The most serious condition inherited in this way is haemophilia, a failure of the blood to clot which gener ally leads to death by haemorrhage. A very common group of characters inherited in this way are the four types of colour blindness, namely protanopia (red-blindness) and its milder form protanomalia, deuteranopia (green-blindness) and its milder form deuteranomalia. The two types of red-blindness, with normal sight, give a series of multiple allelomorphs, as do the two types of green-blindness, but it is not certain whether all five characters are allelomorphic. Among other sex-linked recessives are retinitis pigmentosa (in certain families) and atrophy of the optic nerve.
Autosomal Recessive Anomalies.—Autosomal recessive characters are harder to detect. They do not at first sight appear to be hereditary at all, and are best studied in cases such as deaf mutism where two affected persons commonly marry, in which case all the children are commonly deaf and dumb. However recessive anomalies are vastly more likely to appear in the children of marriages between cousins, or of incestuous unions, than in the general public, since two relations are likely to carry the same recessive gene. Among such anomalies are albinism, some types of retinitis pigmentosa, "day-blindness" or total colour-blindness, deaf-mutism in many families, a number of skin diseases and several diseases of metabolism, including alcaptonuria. Many other anomalies are very probably inherited in a Mendelian manner, for example an inability to become immune to measles or scarlet fever, which leads to suffering from these diseases more than once. Since however this character can only be demonstrated when infection has occurred, it is impossible to be certain of its heredity. Similarly cancer is a recessive character in certain strains of mice, and is at least in some degree hereditary in man, though undoubtedly due in part to external causes.
Other Mendelian Characters.—Among non-pathological physical characters which are inherited in a Mendelian manner should be named the iso-agglutinins. Every human being falls into one of four groups, AB, A, B and o. The corpuscles of group o are not agglutinated by any sera, and can therefore safely be injected into members of any group. Those of group A are agglutinated by groups B and o, of group B by groups A and o, of group AB by the other groups. Inheritance is Mendelian, the presence of A or B being due to a distinct dominant gene, the two perhaps being a pair of multiple allelomorphs. The groups are found in different proportions in different races, and are also pres ent in the anthropoid apes. In western Europe brown- or green eyed persons mostly differ from the blue and grey eyed by a single dominant gene, but other genes are also concerned. The sharp segregation which often occurs among the second and later genera tions of inter-racial crosses makes it clear that Mendelian genes are concerned, but their analysis has not yet gone very far.
Quantitative Characters.—Most of the human characters whose inheritance can be studied in large populations are quanti tative and apparently continuously varying characters such as stature, length of forearm or cranial index are probably due to a number of genes. In these cases Mendelian methods are as yet inapplicable, but much may be learned by statistical means. Such characters show the following correlations between relatives : Brother with brother or sister with sister . . about .52 One parent with son or daughter . . . . about .46 Grandparent and grandchild . . . . about •27 Uncle or aunt and nephew or niece . . . . . about • 2 5 First cousins inter se . . about •15 The law of ancestral heredity, applied to eye and coat colour, states that the coefficient of correlation between the character in an individual and any ancestor distant by n+1 generations is are, where a= about 4 and r= about . Earlier work suggested higher correlations with remote ancestors.
In other words, if allowing for the slight tendency of like to mate with like, we may say that of the total variance ancestry accounts for about 54%, i.e., less than half is due to segregation within a family with the same parents, and very little indeed to differ ences in environment. Putting the matter in another way, if the measured character of the average of the parents deviates from the average of the population by a certain amount, their children will on the average deviate by about 7 5 % of that amount. Thus if a father is 4in. above the average height, and the mother 'in. below, the "mid-parent" is i lin. higher. The average height of children of such parents is just over one inch above the average for their sex. Substantially similar results are obtained for the inheritance of health and longevity, eye- and hair-colour, and other characters. It is important that the correlations are the same for characters such as eye-colour, which are not substantially in fluenced by the environment, and height, which may be, as in the case of rickets. It follows that, in the populations so far investigated, heredity accounted for much more variation than environment.
Mental defect is very strongly inherited, but only sometimes does it follow a definitely Mendelian course. In some pedigrees feeble-mindedness appears to be recessive, but there is no such sharp line between feeble-minded and dull individuals as would be the case were this always so. Probably many genes are concerned.