Introduction to the Science of Sociology

by Robert Ezra Park; E. W. Burgess

Next, it must be recognized that characters, as such, are not inherited. Strictly, my son has not my nose, because I still have it; what was transmitted was something that determined the shape of his nose, and that is called in brief a "determiner." So the second principle is that unit-characters are inherited through determiners in the germ cells.

And finally, it is recognized that there really is no inheritance from parent to child, but that parent and child resemble each other because they are derived from the same germ plasm, they are chips from the same old block; and the son is the half-brother to his father, by another mother.

These three principles are the three corner stones of heredity as we know it today, the principles of the independent unit-characters each derived from a determiner in the germ plasm.

How far are the known facts of heredity in man in accord with these principles? No doubt all human traits are inherited in accordance with these principles; but knowledge proceeds slowly in this field.

As a first illustration I may take the case of human eye color. The iris is made up of a trestle-work of fibers, in which are suspended particles that give the blue color. In addition, in many eyes much brown pigment is formed which may be small in amount and gathered around the pupil or so extensive as to suffuse the entire iris and make it all brown. It is seen, then, that the brown iris is formed by something additional to the blue. And brown iris may be spoken of as a positive character, depending on a determiner for brown pigment; and blue as a negative character, depending on the absence of the determiner for brown.

Now when both parents have brown eyes and come from an ancestry with brown eyes, it is probable that all of their germ cells contain the determiner for brown iris pigmentation. So when these germ cells, both carrying the determiner, unite, all of the progeny will receive the determiner from both sides of the house; consequently the determiners are double in their bodies and the resulting iris pigmentation may be said to be duplex. When a character is duplex in an individual, that means that when the germ cells ripen in the body of that individual each contains a determiner. So that individual is capable, so far as he is concerned, of transmitting his trait in undiminished intensity.

If a parent has pure blue eyes, that is evidence that in neither of the united germ cells from which he arose was there a determiner for iris pigmentation; consequently in respect to brown iris pigmentation such a person may be said to be nulliplex. If, now, such a person marry an individual duplex in eye color, in whom all of the germ cells contain the determiner, each child will receive the determiner for iris pigmentation from one side of the house only. This determiner will, of course, induce pigmentation, but the pigmentation is simplex, being induced by one determiner only. Consequently, the pigmentation is apt to be weak. When a person whose pigment determiners have come from one side of the house forms germ cells, half will have and half will lack the determiner. If such a person marry a consort all of whose germ cells contain the determiner for iris pigmentation, all of the children will, of course, receive the iris pigmentation, but in half it will be duplex and in the other half it will be simplex. If the two parents both be simplex, so that, in each, half of the germ cells possess and half lack the determiner in the union of germ cells, there are four events that are equally apt to occur: (1) an egg with the determiner unites with a sperm with the determiner; (2) an egg with the determiner unites with a sperm without the determiner; (3) an egg without the determiner unites with a sperm with the determiner; (4) an egg without the determiner unites with a sperm without the determiner. Thus the character is duplex in one case, simplex in two cases, and nulliplex in one case; that is, one in four will have no brown pigment, or will be blue eyed. If one parent be simplex, so that the germ cells are equally with and without the determiner, while the other be nulliplex, then half of the children will be simplex and half nulliplex in eye pigment. Finally, if both parents be nulliplex in eye pigmentation (that is, blue eyed), then none of their germ cells will have the determiner, and all children will be nulliplex, or blue eyed. The inheritance of eye color serves as a paradigm of the method of inheritance of any unit-character.

Let us now consider some of the physical traits of man that follow the same law as brown eye color, traits that are clearly positive, and due to a definite determiner in the germ plasm.

Hair color is due either to a golden-brown pigment that looks black in masses, or else to a red pigment. The lighter tints differ from the darker by the absence of some pigment granules. If neither parent has the capacity of producing a large quantity of pigment granules in the hair, the children cannot have that capacity, that is, two flaxen-haired parents have only flaxen-haired children. But a dark-haired parent may be either simplex or duplex; and so two such parents may produce children with light hair; but not more than one out of four. In general, the hair color of the children tends not to be darker than that of the darker parent. Skin pigment follows a similar rule. It is really one of the surprises of modern studies that skin pigment should be found to follow the ordinary law of heredity; it was commonly thought to blend. The inheritance of skin color is not dependent on race; two blonds never have brunette offspring, but brunettes may have blondes. The extreme case is that of albinos with no pigment in skin, hair, and iris. Two albinos have only albino children, but albinos may come from two pigmented parents.

Similarly, straight-haired parents lack curliness, and two such have only straight-haired children. Also two tall parents have only tall children. Shortness is the trait: tallness is a negative character. Also when both parents lack stoutness (are slender), all children tend to lack it.