12.

SOME PRINCIPLES OF MENDELIAN HEREDITY¹

ByIRVINGn/aFISHERn/an/an/an/a

I have sometimes said that eugenics is hygiene raised to the highest power. It is a comparatively new movement, but one which is sweeping over the world with wonderful rapidity, and taking hold of the emotions of mankind in a way that no other movement has ever done, or has deserved to do.

First of all, what is eugenics? Eugenics, as the Greek derivation of the word shows, means the science of right breeding. The word was invented by Sir Francis Galton, of England, to express his ideal of founding a world movement to improve the human race. It was, of course, a colossal ambition, and, at first, almost everybody scoffed. Even today there are comparatively few who realize how immediately practical is this dream of Sir Francis Galton’s.

Eugenics does not mean, as many people at first thought, anything like the old Spartan practice of infanticide. The Spartans tried to develop a strong, physical race according to their ideals, and they succeeded, but they did it in a cruel fashion by ruthlessly exposing children when born. Infanticide has been practiced in many of the barbarous countries of the world, and when eugenics was proposed, many people very naturally imagined this was what it meant. But it does not. Nor does eugenics propose to do violence in any other way to any humanitarian or Christian effort. Eugenics does not mean, as some have imagined, compulsory or government-made marriages. Some people have thought that eugenics was some half-baked scheme to breed the human race as we breed domestic animals, and to make a race of pug-noses or blond hair or blue eyes or any other fancy that some master of ceremonies should conceive. Nor does it mean a reduction in the proportion of love marriages. On the contrary, it means an increase of such marriages. Just as soon as men and women come to see and admire, as in ancient Greece, the ideal of physical perfection, they will fall in love on that basis, as nature always intended that they should. There will be less interference with love marriages through ambition to acquire property or title.

Eugenics is simply an application of modern science to improve the human race. "But," says the skeptic, "that will take millions of years!" Nevertheless, I reassert that it is easily practical to alter and improve the human race and to do so in a very short time.

This is the new optimism of eugenics, and it is based on solid evidence. Until recently no one realized how fast the race could improve if it would. Even Galton himself, when he first proposed eugenics, was under the impression that we inherit from our ancestors in a way which would make possible improvement extremely slow. He put forward as a theory (what we now know to be incorrect) that each child gets half of its nature from its parents, one-quarter from its grandparents, one-eighth from its great grandparents, one-sixteenth from its great great grandparents, and so on indefinitely back, the sum total of those fractions being, when added up to infinity, just unity or the whole inheritance. Instead of such a relation holding true, however, we know that a child inherits something from both parents in relation to every character of body or mind, and that the something which it inherits from its mother, is by its mother inherited from one (not both) of its mother’s parents, and likewise the something which it inherited from the father, was inherited from one (not both) of the father’s parents, and so on in two streams on either side, from each parent backward. Thus each individual today is in respect to any one characteristic (such, for example, as eye-color) simply a combination of two beings in any previous generation. One generation back, it is the two parents from whom he gets his eye-color; two generations back it is two out of its four grandparents, and not the other two at all; three generations back, it is two of its great grandparents and not the other six at all. Consequently, if you can carry back your inheritance to someone who came over in the Mayflower, the chances are a thousand to one that you did not inherit any given character, such as eye-color, from that ancestor at all. In fact, you may have absolutely nothing in mind or body which came from him.

The marvelous laws of inheritance are now being fairly well explained and understood. They were discovered first by a priest named Mendel, in the year 1865. But when he gave his discovery to the world, he found the world was blind and deaf, as it often is, to new discoveries, and it waited until the beginning of the twentieth century, when De Vries and other scientists rediscovered the Mendelian principle, which today is the foundation stone of the science of heredity and eugenics.

We can best understand Mendel’s laws by taking a few concrete cases. The first case is that of an Andalusian fowl. We shall consider the two species, pure bred black and pure bred white and confine ourselves to observing the inheritance of the single characteristic, color. Of course, as long as the black mate only with the black, the children will be black and as long as the white mate with white, the children will be white. But if a white mates with a black, the children will not be either black or white, but blue. All will be blue. But the most interesting facts appear in the next generation, when these hybrid blue fowls mate with black, or with white, or with each other. The original of the cross between the white and the black is an entirely new color, blue, which may be considered a sort of amalgam of black and white. But a cross between the blue and the black will not be any new color, but will be either black or blue—and the chances for the two are even. That is, in the long run, about half of the children of blue and black parents will be blue and about half will be black. None of the children will be white. So also crossing the blue with the white will result in half of the children being blue and half white. Still more curious is the result of mating blue with blue. One might imagine that in this case all the children would be blue, but only half will be blue, while a quarter will be black and a quarter white.

These laws seem strange, but at bottom they are simply the familiar laws of chance, the laws which rule heads and tails in coin tossing. Two parents are like two baskets or bundles of traits from which the child takes its trait at random. In the wonderful play of Maeterlinck’s called "The Bluebird," we are taken to the "land before birth," where the children are waiting to be born, having selected their parents to be. Of course, this is only a pleasant fancy, like the advice of Oliver Wendell Holmes to children to choose good grandparents, but it is a useful fancy which will help us to understand the laws of heredity. The child of the Andalusian fowl takes its color from its two parents on the same principle which operates in a lottery from which it would take two beans, white or black as the case might be, from each of two baskets. Every individual is a sort of basket containing millions of pairs of beans, as it were, each pair pertaining to a particular characteristic. It took one of each pair of these beans from each parent and will give one to each child.

With this picture of a bean lottery before us it is very easy to understand how the colors of Andalusian fowls are inherited. When two black fowls mate, the offspring must be black, because in this case each parent basket contains a pair of black beans, so to speak, so that the child taking one black bean from each basket will necessarily draw a black pair. For the same reason the child of two white fowls must be white. But when a black and white fowl mate, the child takes a white bean from one parent and a black from the other, its own color being a resultant or amalgam of the two, which in the case of the Andalusian fowl makes blue. Since every such hybrid child has this same combination of a white and a black bean all of these hybrids are alike. All are blue. It is important to remember that this hybrid blue is only a sort of mechanical mixture of black and white, and that the black and white are still separate beans, as it were.

But now suppose a hybrid or blue fowl to mate with a white. This means that the child takes from the white parent or basket one of the two white beans and from the blue parent or basket one of the two beans, of which one is white and the other black; the bean taken from the first or white basket must be white, but that taken from the second or blue or hybrid basket may be either white or black. It is a lottery with an even chance of drawing white or black. In the long run half of the children will draw white and half black. Those which draw the white will, since they also drew white from the other parent, be wholly white, but those which drew the black will be blue, since they will have one black and one white bean. We see too that the white child is just as truly white as though it had not had a hybrid parent, for of the two elements or beans which the hybrid carried, the black one was left behind untaken. We see that the blue child is a hybrid exactly like its hybrid parent, and not any new kind of cross between the blue and the white. In short, the children of a blue and white are either the one or the other, and not a mixture. In the same way if a blue mates with a black, half of the children will be black and half blue.

Finally we come to the mating of a blue with a blue. Here the lottery is to pick a bean from two baskets, each basket containing a white and a black bean. When one is taken at random from either of these two baskets there is an even chance that the bean from the father is white or black and an even chance that the bean from the mother is white or black.

Now, what is the chance that the child draws a white bean from both baskets? Evidently it is one chance in four; for there are four ways equally probable in which you can take these beans, viz.: (1) black from the father basket and black from the mother, (2) white from the father and white from the mother, (3) white from the father and black from the mother, (4) black from the father and white from the mother. So the children could draw both white once in four times, both black once in four, and a white and a black in the other two cases. And that is why from two blue Andalusian fowls, on the average you will have one-quarter of the children black, one-quarter white, and the other two-quarters, blue. Again let us stop to emphasize the fact that the black children of these hybrids are just as pure blooded as their black grandparent, and will mate with other pure-blooded black in exactly the same way as though there had never been any white in their ancestry. The white strain has been left behind, or been "bred out."

We have spoken of only one character or characteristic—color. The same laws apply to other characters. Often different characters are inherited quite independently of one another. Each of us is a basket or bundle of very many qualities, each quality being a little compartment of the basket with two beans in it. There is, as it were, a pair of beans for every unit trait, whether that trait relates to color, to musical ability, or to any one of hundreds of other characteristics.

To summarize the laws of inheritance of the unit character called color in Andalusian fowl, we have really six ways in which we can consider the mating of the three colored fowls (black, white, blue): (1) black may mate with black, in which case all the offspring will be black, (2) white may mate with white, in which case all the offspring will be white, (3) black may mate with white, in which case the offspring will all be blue—a hybrid containing both black and white elements, (4) blue may mate with black, in which case half the offspring will be pure bred black, and half hybrid blue, (5) blue may mate with white, in which case half the offspring will be white and half blue, (6) blue may mate with blue, in which case a quarter of the offspring will be white, a quarter, black, and a half, blue.

These results are the fundamental laws of Mendel. But the results are not always clear as in the case of the Andalusian fowl. In that case the hybrids were not like either parent, but were a new color, blue, so that they were labeled at once and recognizable as hybrids—but this is not generally the case. Take, for instance, guinea pigs. What will be the result of mating an "albino" white with a black guinea pig? Quite exactly the same principle applies as in the case of the Andalusian fowl, but the principle is not as clear to see. All the offspring are hybrid, but they will not be blue, they will be black. They will look like the black parent, yet they are different. The black color predominates; i.e., black is "dominant" over white, while the white recedes out of sight, or is "recessive." This hybrid black guinea pig is like the hybrid blue Andalusian fowl. It is a hybrld, a combination of white and black, but in the guinea pig the black covers up the white so that nothing in the color reveals the fact that it is a hybrid. Now if the hybrid black offspring of these black and white guinea pigs mate with each other, the result will follow exactly the same Mendelian law as applied to the Andalusian fowl. But this will not be so clear, because now we have two kinds of black instead of a black and a blue.

How then are we to distinguish between the one pure bred black, the thoroughbred, and the two blacks that are hybrids so that we can be sure which is which? The only way they can be distinguished is to wait and see what their offspring will be in the next succeeding generations. The one that is a thoroughbred will behave like a thoroughbred. For instance, if mated with white it will have nothing but black children. But if one that is hybrid black mate with one that is white, only half of the children will be white; these white children reveal the fact that their black parent was a half breed. Then we can put a tag on that black parent. If proper tags are put on the blacks so as to distinguish between the pure blooded and the half blooded—say a blue tag on the hybrids and a black on the thoroughbreds—we shall get exactly the same results as described in the case of the Andalusian fowl, in the six cases mentioned. The same principles apply to qualities of guinea pigs for characteristics other than color. Thus if a long-haired guinea pig mates with a short-haired guinea pig, all the offspring will be short-haired, because short hair is dominant over long hair. Again, if a smooth coated guinea pig mates with a rough coated one the resulting offspring will be rough coated, because a rough coat is dominant over a smooth coat.

By means of this Mendelian law it is thus possible to predict what will happen in various cases, not only for animals but for plants, and not only for the lower animals but for man himself. Mendel made his experiments mostly with plants. He took garden peas, twenty-two varieties. He crossed them and he found that when he crossed a wrinkled pea with a smooth pea all the children were smooth, but they were hybrids. They did not show any difference from one of the two parents. They showed a difference from the other, but they were hybrids nevertheless. They were not really thoroughbred smooth peas, but they were hybrid smooth peas. Then he mated these hybrid smooth peas with each other and the peas in the next generation were one-quarter wrinkled and three-quarters smooth, but he discovered that of those three-quarters only one-quarter was really smooth in the sense that it would breed true ever after. The others were hybrids and bred just like their parents. Again he took peas which were tall and mated them with peas that were dwarfed and he found that all the children were tall.

In other words, the character of being smooth was dominant and the character of being wrinkled was recessive, while likewise the character of being tall was dominant and the character of being dwarf was recessive.

Again he took peas according to the color of the flower—those that had purple flowers and those that had white flowers—and he found that purple was dominant over white. When the two were crossed, the children would be all purple, but hybrid purple. If these hybrid purples were mated with each other, he found that one-quarter of the next generation would show white again according to the Mendelian law; one-quarter would be thoroughbred purple, and one-half would be hybrid purple. And so he worked with a number of other varieties of peas and other plants.

The various characters of rough or smooth, long haired or short haired, white or black, etc., are inherited independently of one another. That is to say, the child takes from the mystic baskets one pair of beans relative to color, another relative to hair length, another relative to coat, and so on, so that it may be, for instance, long haired and rough coated, long haired and smooth coated, short haired and rough coated, or short haired and smooth coated.

This independent inheritance does not always hold true. Sometimes two traits always go together or always avoid each other. Again, a particular trait may be dominant to another trait but recessive to a third, or dominant in the male and recessive in the female. Each case must be studied by itself, but when the rule is found it can be depended on and used to predict what will happen in other like cases.

These laws are a curious mixture of chance and certainty. In certain circumstances, as we have seen, we can predict with certainty that the offspring will be black, white, blue, or whatever the case may be. In other circumstances we can only state what the chances are. But these chances can be definitely stated as one in two, one in four, or whatever it may be, and where there are large numbers of offspring this amounts to a practical certainty that definite proportions will have this or that color, or other characteristics.

Evidently such definite knowledge can be made useful, and it has been made useful in England. Professor Biffen has created, to order as it were, in accordance with specifications drawn up officially, certain new and valuable species of wheat. This he did by crossing existing species so as to get "hybrids" without the undesirable qualities and with the desirable ones. One species of wheat is resistant to "rust," and another has a stout stalk, another is beardless, another bears a large number of grains on a stalk, another a large yield per acre, but until Professor Biffen created it, no species possessed all these possibilities. By successive crossing of the existing species, however, he finally obtained species possessing all of these desirable qualities. Moreover, the desirable qualities were permanent because the other or undesirable qualities have been "bred out."

The same Mendelian principles undoubtedly apply to the human race, although as yet only a few traits have been carefully studied. Eye color is one of these. Imagine a marriage of a thoroughbred, black-eyed Italian with a thoroughbred, blue-eyed Irish. What will be the result? All the children will be black-eyed, black being dominant over blue; but these black eyes are not the genuine article that the Italian parent possessed. They are a blend, and it is only because the black element dominates over or conceals the blue element that we cannot see on the surface that there is any blue there. But it may come out in the next generation; for, if these half-blooded individuals marry among themselves, one-quarter of their children on the average will be blue-eyed. The other three-quarters will be black-eyed, but only one-quarter will be "really and truly" black-eyed, i.e., black-eyed like the Italian. The remaining half are hybrid black, like the parents. It is only a sort of imitation black, so to speak.

The appearance of blue eyes in the second generation is the long observed but formerly mysterious "atavism," or reversion to the grandparent.

Next, suppose the children of an Italian and an Irish parent intermarry with pure bred Italians. We immediately know what will be the result. All the children will be black-eyed, but among a large number only half will be thoroughbred black-eyed. The other half will be "imitation" black-eyed. The case is just like the mating of hybrid black guinea pigs with thoroughbred black guinea pigs, or of the blue fowl with the black. Similarly if the Irish-Italian hybrids marry with pure Irish, half the offspring will be blue-eyed and half will be hybrid black-eyed.

Black eyes are "dominant" over blue eyes because the black color is due to a pigment, while the blue color is due to the absence of this pigment. In general a quality which is due to the presence of some positive element is dominant over a quality due to the absence of that element. A child inheriting from a blue-eyed person simply draws a blank from that side in the lottery.

The case of skin color in human beings is more complicated. The skin color of an African is, according to the findings of Doctor Davenport, not a unit character but due to four factors. Without going into detailed explanation it follows, and the facts seem to substantiate the conclusion, that (1) the children (mulattos) of a white and a black parent have two color factors, and will all be of the same color midway between the colors of the parents; (2) the children of two mulattos will still be mulatto; (3) the offspring (quadroon) of a mulatto and a white will have one color factor and will all be alike midway between the parents, thus bringing us to a unit character; (4) the children of two quadroons will be quadroons; (5) the children (octoroons) of a quadroon and a white will be all quadroon color, but getting this color from only one side and drawing a blank as it were from the other side, they will be quite different from the true quadroons so that (6) of the children of two octoroons, one-quarter will be white, one-quarter quadroons and a half octoroons, like the parents; (7) of the children of octoroons and white, half will be octoroon and half will be white.

It is to be noted that when a white octoroon appears the black element has disappeared completely so that there is no danger of its reappearance in later generations from marriage with Caucasians. This does not mean, however, that all negro characteristics such as woolly hair, flat noses or thick lips, will disappear.

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