Being Well Born Part 7

Among other normal characters in man, as far as available evidence goes, dark skin is dominant to light skin; normally pigmented condition to albino; and nervous temperament to phlegmatic.

=Digital Malformations.--=An interesting and easily followed defect is a condition known as _brachydactylism_, in which the digits are shortened because of the absence or rudimentary condition of one segment. The fingers, therefore, appear to be only two-jointed like the thumb. Several families showing this defect have been charted and it appears to behave as a typical dominant. In looking over such a chart (Fig. 21, p. 106) one is struck by the fact that only half of the children from most of the matings show the defect, but when we recall that the affected parent, after the first generation, probably carried the condition in only the simplex form and married a normal individual, such a result is just what would be expected (see formula 2).

_Polydactylism_ (Figs. 22, 23, pp. 109, 110) is a condition in which there are extra digits on hands or feet. The character, with apparently slight exceptions in a few records, behaves as a typical dominant. Among other digital defects which are inherited as a dominant is a condition known as _syndactylism_ (Fig. 24, p. 111), in which two or more digits are fused side by side. For an example of syndactyly which seems to be in the cla.s.s of s.e.x-linked characters, see Fig. 15, p. 65.

=Eye Defects.--=_Congenital cataract_ is another not uncommon defect in man which is transmitted as a dominant (Fig. 25, p. 112) with occasional irregularities. It is a condition of opacity of the lens of the eye which produces partial or total blindness. In a paper on _Hereditary Blindness and Its Prevention_, Clarence Loeb (1909) mentions 304 families of which pedigrees have been published. Of the 1,012 children in these families 589, or 58 per cent., were affected. It is obvious that this is near the expected percentage in the case of a dominant trait where matings of affected with normal individuals prevailed. An unfortunate circ.u.mstance about this malady from the eugenic standpoint is the fact that it is frequently of the presenile form which comes on late in life so that it is usually impossible to predict whether an individual of marriageable age is immune or will later become affected.

[Ill.u.s.tration: FIG. 22

Radiograph (Courtesy of Dr. W. B. Helm) showing polydactyly in a child's hand. For genealogy of this see Fig. 23, p. 110.]

[Ill.u.s.tration: FIG. 23

Chart showing a history of polydactylism through five generations in the B---- family. The individual whose hand is pictured in Fig. 22, p. 109, is of the fifth generation. Squares represent males, circles females.]

Another defect of the eye following the course of a dominant in heredity is a pigmentary degeneration of the retina known as _retinitis pigmentosa_. Atrophy of the optic nerve is also involved and the final result is blindness. Still another example frequently cited is that of hereditary night blindness (_hemeralopia_), a disease in which the affected person can not see by any but the brightest light. In most affected families the final outcome is usually total blindness. One of the most remarkable pedigrees of defects in man ever collected is one of this disease published by Nettleship. He succeeded in tracing the defect through nine generations, back to the seventeenth century. The genealogy includes 2,116 persons. The character behaves as a single dominant in males, but frequently, though not always, females may be carriers of the defect in transmissible form though not exhibiting it themselves. That is, males in which the condition is simplex (Aa) develop the defect but females of similar simplex const.i.tution (Aa) frequently do not. It follows, therefore, that normal males of such strains will have normal offspring but normal females may have affected children.

[Ill.u.s.tration: FIG. 24

Radiograph (Courtesy of Dr. W. B. Helm) showing a partial syndactyly in each hand of an individual. Some degree of webbing between the more distal portions of the affected parts is usual.]

[Ill.u.s.tration: FIG. 25

Pedigree of a family with presenile cataract (black symbols); numbers in circles indicate unaffected individuals (after Davenport).]

=Other Defects Inherited as Dominants.--=Not to go into details other defects which behave as dominants or modified dominants in human inheritance may be mentioned. The following list is not complete and it must be understood that in some cases the statistics are insufficient to justify us in making anything but a tentative decision. We may thus enumerate as dominant over normality: _Achondroplasy_ (abnormally short limbs with normal head and body); _Keratosis_ (thickening of epidermis); _Epidermolysis_ (excessive formation of blisters); _Hypotrichosis_ (hairless, toothless condition); _Diabetes insipidus_; _Diabetes mellitus_; ordinary (not Gower's) _muscular atrophy_; _Glaucoma_ (internal swelling and pressure of eye-ball); displaced lens; _Coloboma_ (open suture in iris); spottedness of hair-coat; and corneal opacity.

As a final ill.u.s.tration of a serious malady in man which acts as a dominant in inheritance, let us take _Huntington's ch.o.r.ea_. Ordinary _ch.o.r.ea_, or St. Vitus' dance, a disorder characterized by involuntary muscular movements, is commonly though not always confined to children and usually ends in recovery, but _Huntington's ch.o.r.ea_ appears typically in middle life and is a much more dangerous malady. Fig. 26, p. 114, represents the family history of one of five cases which have been studied by Doctor Lorenz in the Mendota Hospital for the Insane. All charts which have been platted of this malady show it to be inherited as a dominant.

This means that half of the children of an individual who carried the malady in the simplex condition, and all the children of one who carries it in the duplex condition, are probably marked for this terrible end. And the true horror of it can only be appreciated by one who has seen the last stages of the malady. The victim once in its grasp gradually becomes wrecked in mind and body; the muscular twitchings and disorders of movement continually increase and dementia progresses until at last death ensues. Fig. 27, p. 115, is another chart showing inheritance of _Huntington's ch.o.r.ea_. In still a third case at the Mendota Hospital, the gravity of the situation can be appreciated when one realizes that the patient is the father of ten children, ranging in age from one to seventeen and one-half years. The calamitous fact that this disease does not manifest itself usually until middle life makes it likely that these children will all reach maturity, marry and in turn probably produce offspring before the doomed members of the family realize their fate.

[Ill.u.s.tration: FIG. 26

Chart showing descent of _Huntington's ch.o.r.ea_ in the P---- family (courtesy of Dr. W. F. Lorenz). Squares represent male, circles female; shaded figures are ch.o.r.eic members of the family; partially shaded figures, slightly affected or very "nervous" members. The members of the last generation are for the most part still too young to show their condition. The cross indicates the individual in the asylum from whom the record was traced back.]

CASES OF RECESSIVENESS IN MAN

=Recessive Conditions More Difficult to Deal With Because They Are Frequently Masked.--=Coming now to the question of recessive conditions in man, we find that defects are more likely to be of recessive than of dominant type. Apparently normality usually means the presence of normal determiners and abnormality, the absence of some essential determiner. In the latter case, a unit-factor has seemingly been lost out in some way in the germ-plasm, and the product of such germ-plasm is therefore incomplete. As long as the loss is counterbalanced by the presence of a single determiner from the other line of ancestry, that is, as long as the simplex (Aa) condition prevails, the loss may not be in evidence, except in cases of incomplete dominance (taints, etc.), but any mating which permits of the production of the nulliplex condition will bring the defect to expression again.

[Ill.u.s.tration: FIG. 27

Chart showing inheritance of _Huntington's ch.o.r.ea_ in the R---- family (courtesy of Dr. W. F. Lorenz); 1, 2 have been patients at Mendota Hospital for the Insane; 3, died of "paralysis"; the fourth or last generation indicated by the cross, ranging in age from 6 to 14, are too young yet to show their condition as regards this malady.]

The obscure nature of recessives makes such conditions more difficult to deal with than dominant defects. For as regards the latter we have seen that marriage of unaffected members of the family as far as that particular trait is concerned, is perfectly safe, even to a cousin, for once the germ-plasm is purged of such a positive factor, it, in so far as we know, remains pure. But in the case of a recessive character due to the absence of some necessary determiner a normal offspring of simplex const.i.tution (Aa) will probably transmit to half of his children the capacity for handing on the defect, or if mated to another normal individual of simplex const.i.tution (Aa) is likely to have the actual defect revealed again in one-fourth of his children and latent in two-thirds of the remainder.

=Albinism a Recessive.--=As an easily understood ill.u.s.tration of this type of case we may take human albinism, a condition which is due to the absence of a pigment-developing determiner. According to Davenport the albinic condition is recessive to normal condition. If albino (aa) is mated with albino (aa) nothing but albino children may be expected. An albino (aa) mated with a normal individual will have normal offspring (Aa), but they will have the capacity for transmitting albinism to their descendants. Thus the normal offspring (Aa) of an albino (aa) and a normal parent (AA) if mated to another normal individual (Aa) who has also had an albino parent will probably transmit actual albinism to one-fourth of his children and the same capacity that he himself has of producing albinos, to one-half of his children, although the latter will appear to the eye to be normal.

=Other Recessive Conditions in Man.--=If for albinism we subst.i.tute certain forms of insanity, hereditary feeble-mindedness (Fig. 28, p. 118), or hereditary epilepsy, all of which apparently follow the same law, we can readily understand how unfit such matings are where both strains are affected. Marriage with similarly defective stock will result in the affection appearing in one-fourth of the progeny, and one-half of them, though apparently normal themselves, will have the capacity for transmitting the imperfection. It is in the existence of such hidden factors that the chief danger in the marriage of cousins, or in fact any consanguineous marriage lies.

A few of the various defects which seem to be inherited as recessives when mated with normality are: susceptibility to cancer; _ch.o.r.ea_ (St. Vitus'

dance); true dwarfism (all parts proportionately reduced); _Alkaptonuria_ (urine darkens after pa.s.sage); alcoholism and criminality, where based on mental deficiency; hereditary _hysteria_; _multiple sclerosis_ (diffuse degeneration of nervous tissue); _Friedreich's disease_ (degeneration of upper part of the spinal cord); _Merriere's disease_ (dizziness and roaring in ears); _Thomsen's disease_ (lack of muscular tone); hereditary _ataxia_; possibly the tendency to become hard of hearing with increased age; and possibly, non-resistance to tuberculosis.

Of non-pathological conditions in man which are inherited as recessives, apparently either very great or very small intellectual ability are examples.

[Ill.u.s.tration: FIG. 28

Chart showing descent of feeble-mindedness as a typical recessive (after G.o.ddard). Squares represent males, circles females; DD, h.o.m.ozygous dominant; DR, heterozygous dominant (i. e. normal although a carrier); RR, pure recessive; N, normal; F, feeble-minded; A, alcoholic.]

=Breeding Out Defects.--=Even though recessive defects occur in a stock, there is the possibility of diluting out the imperfection in successive generations if care is taken always to marry into a stock wholly free from it. For example, a normal individual carrying a recessive defect will bear the abnormality in half of his or her germ-cells. This means that when such an individual marries a normal, non-carrier, half of their children will be wholly normal (AA) and half will be carriers; normal but of simplex const.i.tution (Aa). If now this generation, carriers and non-carriers, marry only into normal strains of duplex const.i.tution, then their combined issue will be likewise normal with only one-fourth of them carriers of the imperfections. This means that even if all of this last generation were married to persons having the defect only one out of four would have children showing it although the remaining children would be carriers. On the other hand if mated to normals only one-eight of the next generation would be carriers. Thus by continually marrying into strong strains liability to manifest any recessive defect can be diminished in a few generations until the descendants are no more likely to have defective children than are members of our ordinary population.

The proportion in which the recessive defect would appear in successive generations if all persons in a given generation married only normal individuals who were non-carriers is indicated in the following table where AA indicates a normal individual, Aa one who is normal but a carrier, and aa an individual with the imperfection expressed; to indicate proportions simply after the first generation, four is arbitrarily chosen as the number of children which results from each marriage:

Matings Children Generation 1 aa AA = Aa

Generation 2 Aa AA = 2AA + 2Aa

Generation 3 AA AA = 4AA AA AA = 4AA Aa AA = 2AA + 2Aa Aa AA = 2AA + 2Aa ---------- 12AA + 4Aa

=Other Inheritable Conditions in Man.--=While many pedigrees show beyond dispute that such qualities as musical ability, literary ability, memory, calculating ability, mechanical skill, longevity, peculiarities of handwriting, obesity and muscular strength, for example, are inherited, their modes of inheritance have not yet been sufficiently a.n.a.lyzed to express them exactly.

CHAPTER V

ARE MODIFICATIONS ACQUIRED DIRECTLY BY THE BODY INHERITED?

=Which New Characters Are Inherited?--=Any new feature which appears in a given organism may have had its origin in some change which has come about in the germ from which it sprang, or it may be merely the product of some unusual stimulus operating on the body. While the outcome, as far as the present individual is concerned, is in each case a definite modification, the matter of inheritance is a very different question. On the first alternative where the new character is the outcome of germinal change, it is obvious that the altered germ-plasm will find expression in a similar way in succeeding generations as long as the new germinal combinations persist. On the other hand, if the new character has resulted merely from some influence operating on the body of the individual, then to be inherited it would also have in some way to be transferred to and incorporated in the germ-plasm. Inasmuch as the body or soma of any individual is highly plastic and since various of its ultimate features may be mere somatic modifications, it is important to decide if possible whether or not somatic variations which are not of germinal origin can be inherited.

=Examples of Somatic Modifications.--=For example, the small foot of the Chinese woman of certain caste is the result of inherent germinal factor for the production of a foot plus the effects of binding which are in no wise germinal. The hand of the skilled pianist is a normal hand of germinal origin and normal environment plus the effects of special training. Again, the head of the Flathead Indian is a normal head of germinal origin and environment plus the effects of flattening. Similarly, almost any malformation of extrinsic origin may be cited, ranging from mutilations and amputations, scars and the like to monstrosities such as one-eyed fish which may be produced by subjecting a developing embryo to adverse conditions of development.

=Use and Disuse.--=Even reactions set up through the organism's own activities must produce changes. For example, a muscle has a certain average of normal development in the average man; it comes to this through the innate nature of its component cells plus a certain average amount of exercise. It may, however, be developed far beyond this average by excessive exercise. On the other hand, it is a well-known fact that an unused organ weakens or may remain but partially developed. Thus either use or disuse may play an important part in the molding of a given individual. But whether or not in doing this it similarly affects the germ is a very different matter.

=The Problem Stated.--=The question is can such enhanced or suppressed development, or can new or modified characters, produced in an individual by external agencies be so reflected on the germ-cell of the individual that they tend to reappear _as such_ in its offspring without requiring the same external factors for their production?

=Special Conditions Prevail in Mammals.--=Before proceeding further we must recognize clearly the very special conditions which exist in most mammals. With them environment is in part an intra-maternal environment and in part independent of parental influences. Thus the formula for most non-mammalia would be--