The mechanism of heredity

In all the many-celled forms of life, some cells have achieved specialized functions which involve the loss of their primitive capacity to yield new life forms; the task of carrying on the stock is the property of a restricted group of cells, the germ cells. Body cells and germ cells are distinct. The germ cells are nourished and protected, but, so far as is known, not greatly influenced by the vicissitudes affecting the life of the body. They are capable of transmitting the ancestral traits from generation to generation regardless of crafts acquired, languages spoken, prejudices cultivated by the people who carry them.

Germ cells, however, play their role in the changing, as well as in the continuing, of life. Preparatory to the reproduction process, in which a germ from the father and a germ from the mother unite, each germ cell has undergone a process which throws out half of its germinal potentialities. This "reduction division" yields two gametes, each of which has half the number of chromosomes which the germ cell had before; thus, in man, there are 24 instead of 48. When the fusion of male and female germ cells occurs, the original number, 48, is reconstituted by sheer addition. This process means, however, that of the total 96 available chromosomes in the two parents, there can never actually be more than 48 in the new individual; and which ones they are, so long as there are 24 from each parent, is determined by such infinite biochemical complexities that we have to treat them as occurring at random. Two children with the same parents may be very much alike or they may be very different indeed, depending on similarity or difference in the germinal constitution which they have received; the striking difference between two such children thus occurs not in spite of heredity but because of it. Around a common core of biological similarities in any stock there are the endless individualities traceable to unique combinations of genes. (New genes--mutations-also appear; although they are studied in the fruit fly as soon as they appear, their role in man is practically unknown.)

Another problem relating to gene structure that has taken on importance recently is the likelihood that groups of genes located on the same chromosome, linked together and remaining linked through the formation of the new individual, may be traceable in the individual constitution. Burks succeeded in identifying, through life history data and direct clinical examination, the linkage of certain hair colors with certain tooth conditions such that the members of a family who have this hair color have the same peculiar tooth structure. With techniques established through these somatic studies, she laid a foundation for investigating the linkage of psychological peculiarities with physical peculiarities. The incidence of the manic-depressive psychosis, she suspected, may in some families be linked with identifiable idiosyncrasies of the teeth. As always in such studies, one abstracts a relation from a maze of particulars. Patient care and mental hygiene may help the person with a germinal weakness, just as dental care and orthodontia can. But the importance of such linkage lies not in the fact that anomalies become more controllable, but in the great likelihood that functional unities, genuine elements in personality structure will be revealed. Whatever clings consistently together with a somatic trait, or stems from a common origin with it, has a chance of being "elemental." Since distinct personality features are scarcely likely to be the direct results of such characteristics as nonerupted teeth, and since, nevertheless, the two characteristics cling together from one generation to the next, it is likely that the specified psychological trait has a simple genetic structure. Again we say that this structure is ultimately a biochemical disposition; it is realized only in the process of growth and interaction with the environment; and if it is pathological, therapy may remove it. The fact that a man shaves, although making the face as smooth as one pleases, does not affect the situation confronting one's adolescent sons; the germinal dispositions go on generation after generation.

Of all the verbal quagmires into which man has fallen in his attempt to tell what he is, none has caused more damage than the uncritical use of the opposing terms "heredity" and "environment." Our folklore is saturated with the belief that we inherit certain full-fledged traits and that we acquire other traits by virtue of environmental forces. For three-quarters of a century the literature on "nature and nurture" seemed to support such a belief, and authorities are still quoted to show that some traits are truly hereditary, others truly acquired. The toughness of this form of thinking, its resistance to evidence, is shown by the fact that modern geneticists, embryologists, comparative psychologists, and students of the infant and small child, though perfectly aware of the fallacies in this type of language, find themselves forced, if they are to make contact with their readers or hearers, to employ these question-begging terms.

But if the organism is a tissue system undergoing changes partly because of its own dynamics, partly because of interaction with the outer world, it is "acquiring" new characteristics all the time, never by accretion but always by modification of what it is. No organism differently constituted could acquire in the same way or acquire the same tendencies. What is acquired is just as completely an expression of the inherited make-up of the organism as it is an expression of the outer forces. Nothing meaningful can be said about our acquisition of tendencies, or about the effects of the environment, except in terms of a specific knowledge of the dispositions of the living system. New tendencies, habits, traits are not acquired, plastered on, or stuck on as one affixes a postage stamp to a letter. The organism grows into new phases of behavior, under one form of pressure or another, as long as life continues. Similarly, to use the term "acquired traits" is to talk redundantly, for all traits are acquired by some kind of reaction with environmental forces. There was a time when any given trait was not there; and if one speaks precisely, no trait that is there will remain long in its present apparent form. As Heraclitus said, no man can step twice into the same river; it is a different man, a different river. The way a man talks, or even his attitude toward himself, basic as it may be, is a function both of past tissue changes and of present environing pressures.

If it is true that nothing is acquired in the popular sense, it is equally true that nothing is inherited. As a result of very complex interactions--each of two cells merging its field dynamics with that of the other cell--a new life system involving its own field relationships is established; in it some features of each of the two earlier life systems are still recognizable, but it has a large number of new dispositions which never existed before in any creature on the face of the earth. Hence even the near-identities between the offspring and the parent are not instances of heredity in the popular sense. Since it is not the bodily characteristics of the parents that are transmitted, but only the germinal dispositions, and since organisms show wide differences from their parents in bodily characteristics, the word heredity refers at best to the continuity of certain dispositions from one generation to the next. Even in this sense, however, the term causes a great deal of trouble because dispositions as such cannot be observed. It is the dispositions as realized, as fulfilled by certain environing circumstances, that we can study. There develops the paradox that heredity is known only by the liberation of the hereditary potentials through specific environmental forces; and what is liberated is as much a function of the environing pressures as it is of the latent or potential dispositions.

In the life process, whether studied in embryology or later, nature is not made up, mosaic fashion, of hereditary and environmental elements. The terms heredity and environment serve no purpose for referring to methods of classifying types of behavior, and they can profoundly damage the whole effort in personality study. What they really denote, if carefully studied, is a dual function; the two always occur together and are separated only for conceptual convenience, as we might, for convenience, define two interacting chemical reagents as if one were the agent and the other the substance acted upon.

Defining the environmental pressures as those observed by the physical sciences in terms of light, temperature, acidity, etc., we shall reserve the term heredity for the dispositions of the organism which allow these physical forces to produce greater or lesser changes in it or to manifest this or that type of qualitative variation in its response pattern. Heredity, then, will be the system of predispositions, throughout the life history of the individual organism, which, when different organisms are compared, is responsible for the varying effects of known environmental pressures. In this sense, a birch tree and a man differ in heredity in that they chronically react differently to sunlight and water; and so far as two human beings in the same sense. chronically (from the germ cell on) react differently to sunlight and water, for example, they are different in heredity. Genes and chromosomes are neither a miniature of the later adult life form nor a prophecy of what it is to be. They are simply keys to potentials for differential responses which will ultimately appear under environmental pressure.