( Originally Published 1923 )
Heredity.—Although the fact of resemblance between parents and offspring has been known and utilized by civilized man for many centuries, it is only within the past generation that enough has been learned of the mechanism of reproduction and development to enable us to explain some of the outstanding facts.
In reproduction without sexual union we can see that one generation is like another because both are parts of the same protoplasm. In bisexual reproduction, how-ever, the continuity is not so apparent. The germ plasm (see page 83), which is set apart from the developing mass of cells shortly after the embryo begins its development, produces the gametes. But what happens when the gametes, or the germ plasm, from two different individuals unite?
In every cell there is a portion of the protoplasm which is denser than the rest, and this is called the nucleus. With the aid of powerful microscopes we can see within the nucleus portions that are chemically different from the rest, and that undergo visible physical changes during the process of cell division. There is a definite number of these portions in each nucleus, usually an even number; they are called chromosomes. The number of chromosomes is the same in all the cells of the body of a given species, although it varies from one species to another. An exception to this statement has been worked out for many species of animals, including man, within a comparatively few years—an exception of great significance, and one that helps toward an understanding of the de-termination of sex. The number of chromosomes in the cells of a male human being is 47; in the cells of a female, 48.
At the time of formation of gametes, both in the male and in the female, the number of chromosomes becomes reduced, so that each germ cell has only one half of the number that the body cells have. Thus while the body cells of a female human being have 48 chromosomes each, the egg cells have only 24 each ; and while the male individual's cells have 47 chromosomes, the sperm cells are of two kind, one bearing 24 and the other 23 chromosomes. When the two germ cells unite at the time of fertilization, the number of chromosomes in the zygote (fertilized egg) is again the same as that in the body cells.
It has long been suspected that there is something in these chromosomes that determines the later appearance of the various traits or characteristics which make up the species, and which distinguish one individual from another. In recent years it has been possible, by means of experiments with various plants and animals, to locate rather definitely in certain portions of these chromosomes the determining factors for a number of characteristics. These determining elements, whatever their constitution, and whatever their mode of action may be, come in equal numbers from the two parents, one pair being concerned, as it were, with each trait.
Certain characteristics have been definitely traced from generation to generation so that their mode of inheritance is sufficiently well known to enable plant and animal breeders to produce strains with almost any desired combination of characters. Thus, among rodents, pigmentation in the hair appears in every individual if one of the parents was a pure-bred black or red, and the other a white or albino, just as it does if both parents are pigmented ; short hair will appear in every individual if one of the parents was a pure-bred short-hair and the other a long-hair, and so on. The peculiar way in which combinations derived from two or more lines of ancestors break up and become scattered among different individuals of the same family can also be explained by the behavior of the chromosomes and their elements during the formation of the germ cells and during the recombination at the time of union or fertilization.
These studies indicate that the "inheritance" is precisely the same from both sides of the house, although in any particular child certain traits of the father's side may be more conspicuous than the corresponding traits of the mother's side, or vice versa. Thus, brown eyes are said to be dominant over blue eyes. In the union of a blue-eyed person with a brown-eyed, we should expect all of the children to have brown eyes. But we know that in such families one or more of the children may have blue eyes. The explanation lies in the fact that the brown-eyed parent comes from a mixture of stocks; there will be found among his immediate ancestors one or more blue-eyed persons, and in a certain number of cases the germ cells will contain the factor which determines blue eyes. Now when two germ cells from two parents contain respectively, the factor for blue eyes and the factor for brown eyes, the child will have brown eyes because the pigment will be formed, if only one of the chromosome elements for brown eyes is present. But if both germ cells contain the blue-eye factor, then the child will have blue eyes. The offspring of blue-eyed parents will not have brown eyes, but among the offspring of brown-eyed parents there may be individuals having either kind of eyes—unless both parents are "pure-bred" for brown eye color.
The same principles apply to a large number of physical characters in man, and have been established for hundreds of characters in many species of plants and animals. They are probably also true for various mental or emotional traits, which are dependent upon structural and chemical properties of the body. Feeble-mindedness, for example, is probably inherited in the same way as blue-eye color, and various mental traits are known to "run" in families.
Since these characteristics are determined by something that is already present in the germ cells, it is evident that nothing that happens after fertilization can either add to or take away from this inheritance. The training which the mother undergoes, or the music to which she listens during pregnancy, may keep her cheerful or add to her health or satisfaction, but it is not going to modify the chromosomes of the embryo or the characters with which the child will be born. On the other hand, neither will being frightened by a mouse or a fire have effect upon the characteristics of the child.
To be sure, people will continue to report strawberry-marks and moles and dimples and deformities in new born infants whose mothers had experienced some unusual shock or excitement during pregnancy; but no rational connection has as yet been established between these shocks and accidents and the physical or mental peculiarities which they are supposed to cause. Most children are born without such birthmarks or deformities notwithstanding the vicissitudes through which their mothers pass ; and numberless records are available, of perfectly normal children born after most difficult or distressing pregnancies. Moreover, all experiments with lower animals, performed to discover some connection between maternal impressions or experiences and effects upon offspring, have given negative results.
A similar problem is that of the effect of sickness, deformity, training, use or disuse of various organs, and other modifications of the individual, upon the subsequent progeny. The blacksmith develops strong muscles through his daily work; will his children have stronger muscles than they would otherwise? The mathematician becomes. very skillful in the manipulation of numbers; will his children be better mathematicians than they would have been had the father devoted himself to horticulture or preaching? It is likely that the blacksmith's children will have good muscles because they come from a strain in which muscular development comes easy and in which, therefore, some individuals become black-smiths. It is likely that the children of the mathematician will be more mathematical than the children of non-mathematical parents, because they come of stock in which there is present capacity for mathematical thinking. But it is altogether a mistake to attribute the health or the strength or the skill of children to the training or exercise or pains their parents took to improve themselves beforehand. Such effects are ruled out not only by what we know of the mechanism of reproduction and inheritance, but also by the results of experiments especially designed to discover the relation between bodily modification and the characteristics of subsequent offspring.
We must not overlook, however, the possibility of injuring or perhaps otherwise modifying, the chromosomes of the germ plasm either before or after fertilization. Experiments have shown that exposure to extremes of temperature and to various chemical agents can produce profound changes in the germ plasm. Extremes of temperature are excluded in the case of warm-blooded animals, since the temperature within the body remains constant under a wide range of conditions. But it is possible for alcohol and other drugs, and the by products of disease germs to affect the germ plasm directly, just as it is possible for poisons to affect the protoplasm in other parts of the body. The precise way in which these "racial poisons" work is not known; but that they do work to deteriorate or deform the germ plasm is beyond question. At the same time, the injury to the germ plasm will show itself in ways that are without direct relation to the injury which a body may receive from the same agency. It is, therefore, impossible to transmit to the offspring the particular modification which the parent has undergone, whether favorable or unfavorable, whether physical or mental, whether due to mechanical or to chemical influence.
The chief concern of a prospective parent has to do with his own inheritance and health, and with the inheritance and health of the other parent. And once fertilization has taken place, the pregnant mother should be chiefly concerned with her mental and physical health. The combination of germ plasm from two lines of ancestors has determined the sex and all the other basic characteristics of the offspring beyond recall.
But as children continue to be born with an inheritance that is fixed, the immediate problem in every case is to provide for each child such conditions for growth and development as will insure the fullest use of the native capacities, to make the most of the inherent possibilities. That is the first responsibility of parents.