Washington Academy of Sciences 9: 179-187 (1907)
Heredity and Mendel's Law
Charles B. Davenport

FROM its first appearance on earth the living substance has moved like the growing roots of a tree from a single point, without break in its continuity, but ever branching out in new directions. This fundamental stream is the germ-plasm. At intervals, as roots may send up shoots, so the germ-plasm produces the individual body or soma, whose primary purpose is to nourish the germ-plasm and thus assist its further progress.

The soma is important to the student of heredity because it is the index of the composition of the particular germ-plasm from which it arose. The study of somas shows that those arising from the same part of the germ-plasm are very similar; those from distant parts of the germ-plasm are very different; those nearer the starting point of any root are simpler in structure, while those arising more recently are more complex. These differences of the soma prove that the germ-plasm changes in structure and tends to become more complex. These changes in the germ-plasm constitute evolution, and the study of the laws of change of this germ-plasm and the artificial control of such change constitute at the same time the largest and most fascinating work of the modern biologist.

Inside the germ-plasm complex movements are going on. In the first place, the development of a soma at any point is usually preceded by the union of bits of the germ-plasm coming from different parts of the same motor stream, or even from two different streams, recently separated. This is union of sex cells, or gametes, in sexual reproduction. As every part of the germ-plasm is undergoing change, such unions bring together more or less dissimilar germ-plasms. Of the effect of such unions, we get some notion by studying the somas, or offspring that result. If the uniting germ-plasms are quite dissimilar the behavior of the union, as revealed in the soma of the hybrid offspring, is correspondingly interesting and significant.

A second sort of movement in the germ-plasm is molecular. Under certain, still unknown, conditions the composition of the germ-plasm changes and such changes reveal themselves to us as sports or mutations. These occur so irregularly and uncommonly that their study is difficult; but it is one of the aims of experimental work in evolution to initiate and control such changes.

This evening I propose to speak only of the first kind of changes in the germ-plasm — namely, those due to combinations of two dissimilar sorts — and the effect of such combinations on the stream of germ-plasm that issues from the union. The laws of such combinations and their transmission constitute the science of heredity.

In analyzing the phenomena of heredity we must first conceive that the germ-plasm is composed of many units, such as Darwin named pangenes and Weismann determinants. Our knowledge of such units is confined to their representations in the soma — the unit-characters of organisms.

In studying heredity, we focus attention on the behavior of the unit-characters of the soma as the index of the changes in the germ-plasm. Practically, one breeds together two individuals having contrasted characters and observes the corresponding characters in the next somatic generation.

The results of such experiments are in the highest degree interesting and important. They are not yet predictable; indeed, they do not all follow a single law, but in the midst of the diversity of the results a unity is dimly visible. I propose now to cite typical examples of such inheritance in contrasted characters, illustrating the different modes of inheritance.

First, let us look at a case of color inheritance. If a bird having black feathers, like a Minorca, or one having black feathers mottled with white, like a Houdan, be crossed with a white bird, like the White Leghorn, the male hybrids are pure, or almost pure, white. White dominates more or less completely over black; but the black is not lost, it is merely overshadowed, for if two of these white hybrids be mated, about 25 per cent. of their offspring will be as black as the original parents. Such black grandchildren derived from two white parents are known as extracted blacks. The black color has been released from the dominance of the white; and if the extracted blacks be bred together they behave like a pure black race. A characteristic like black which recedes from view in the first hybrid generation, which may be extracted from the dominant type in 25 per cent, of the second generation, and which, after extraction, breeds true, is called a recessive characteristic.

The discovery of the law of inheritance in the case of dominant and recessive characteristics was made forty two years ago by an Austrian monk, Gregor Mendel, while experimenting with plants in his cloister garden. Mendel not only discovered his law, but gave a satisfactory cytological explanation of it. He states that the germ cells of the hybrid must divide into two kinds, each pure in respect to two opposed characteristics, the dominant and the recessive. When two of these hybrids are mated there are four equally probable events: an egg containing the dominant character D may be fertilized with a D sperm or an R sperm, and an R egg may be fertilized by a D sperm or an R sperm. Whenever the character D enters into the combination, that character dominates in the offspring; but where it does not enter, R shows itself in the same. The former event occurs in three-fourths of all cases; the latter in one-fourth, consequently only one-fourth of the offspring are recessive. This is in brief outline the essence of the Mendelian doctrine. It rests on the foundation of purity of the germ-cells and hence purity of the one-fourth of the second hybrid generation that is produced by the union DD and the one-fourth resulting from the union RR. These portions, inbred, should yield only the dominant type and the recessive type respectively. These expectations have been now so often realized that Mendel's law has gained deserved fame as the most important law of inheritance yet enunciated.

Many illustrations of this law are given by poultry. Thus when frizzled fowl are crossed with non-frizzled, the offspring are frizzled; from such frizzled hybrids 25 per cent. plain-feathered are derived. These plain-feathered birds have not, in 50 offspring, thrown one frizzled fowl.

When the silky fowl is crossed with plain-feathered fowl, all of the offspring have plain feathers. When the hybrids are mated 25 per cent. silky offspring result. Such silky offspring breed true. And so I might continue with a dozen illustrations drawn from my experience demonstrating the truth of Mendel's law.

As the truth of a new law becomes apparent, enthusiasm for it is apt to carry one to extremes — to declare its universality and to overlook exceptions. It is well to cultivate the skeptical frame of mind and to remember that what looks like the whole truth may be only partial and that an examination of the exceptions may lead to larger generalities.

First of all we find that even in cases of Mendelian inheritance in the second hybrid generation, the recessives that are supposed to come from two pure recessive gametes show in their somas traces of the dominant type. Thus when a crested bird is crossed with a plain-headed one, and the crested hybrids are then crossed inter se the extracted recessives of the second hybrid generation are plain-headed to be sure, but they show a disturbance of certain feathers. Again foot-feathering, or booting, is dominant, but in the cross between the Dark Brahma and Minorca small feathers can frequently be found on the feet of all the second hybrid generation where 25 per cent. bootless is expected. The boot character is not absent from the 25 per cent., but it is immensely reduced in expression as compared with the booted ancestor.

Second, we find that the two opposed unit characters are not always respectively dominant and recessive. Various classes can be recognized. In some cases both characters appear in the first generation side by side — neither dominating. Thus, when the white Cochin is crossed by the red-black Tosa, the first hybrids are barred with white; in the next generation, to be sure, some white birds are produced, but these frequently show some yellow or buff; also red and black birds, but these have often a washed-out color. In this case there is no dominance and the germ-cells are incompletely pure. This combination is very common.

Another case is that sometimes seen when a black and white are crossed. Instead of white dominating (as is usually the case) the two colors seem to have equal potency and a fine mosaic occurs — the blue. This is almost a blend. When such blues are mated together one gets some blues, but also some black and white mottled in all proportions. Not only is potency of the two characteristics equal in the first generation, but the characteristics appear in all of the germ-cells and reappear in the second hybrid generation in the most varied combinations.

Again, a peculiar case of inheritance was revealed by some experiments made by Darwin. He crossed a White-faced Black Spanish with a pure white silky, which (except in plumage) is a melanic sport. The chicks were blacker than any Spanish chicks, or any other that I know. They continued black until as they grew to maturity the males gained red-laced hackles, saddles, and back-feathers and a broad red wing bar, like the jungle cock. For Darwin, this was a case of reversion, due to hybridization. But that it was not the consequence of hybridization per se, but of this specific cross, is evident from the fact that you do not get a similar hybrid except by crossing a black bird either with a silky, or a pure buff — both of which are devoid of black pigment. Moreover, the result differs according to the black races crossed. If a black Minorca is used, which represents the immediate ancestor of the Spanish, the red in the progeny is much reduced. If a black Game is used, which represents a comparatively recent blackening of a red and black bird, the progeny have much more red.

Still another case. Some fowl have five toes instead of the normal four. When a four- and a five-toed fowl are crossed the offspring show no blend, they are either four-toed or five-toed, and neither character dominates. If the extra toe is well developed in the parents a large percentage of the offspring have an extra toe; if the extra toe is poorly developed in the parents a small percentage of the offspring have an extra toe. In this case we have a unit character — extra toe — but no dominance or recessiveness and no evidence of pure germ-cells. The result seems to depend on the relative potency of the four-toed and the five-toed tendency.

Third, we find that one of the two opposed unit characters may be dominant in certain individuals and their offspring forming a dominant strain — the same character may be recessive in other individuals or strains. Thus when high nostril and low are crossed, the offspring have a low nostril, but in one case out of a hundred an individual gamete carrying high nostril prevailed over the gamete carrying low nostril and the offspring had a high nostril. Again I bred a single comb to a rose comb (rose being dominant) and, in the second hybrid generation, obtained some extracted single-combed birds. These inbred gave a hundred single combs and one rose comb; the ordinarily impotent rose comb determinant of the extracted singles here overpowered its mate.

But the most striking illustration of the potency of a strain appeared last summer. Two years ago I mated a tailless Game cock, No. 117, to some tailed white Leghorns and got 100 per cent. tailed birds. No. 117 is a bird imported from England; it was exhibited at the Louisiana Purchase Exposition, St. Louis, 1904, and took a prize. Last summer I mated together these tailed hybrids, expecting to get 25 per cent. tailless birds; of about 100 offspring none was tailless. I also crossed some of the tailed hybrids with their father — No. 117 — expecting to get 50 per cent. tailless. Of 88 offspring none was tailless. Altogether, out of 200 offspring of this tailless cock, where I expected 90 per cent. tailless birds, I got not one. On the other hand, using some of the same hens with another cock (the son of No. 117), from 50 offspring, where I expected 25 tailless, I got 24 tailless. In No. 117, although tailless, the tailed tendency strongly dominates over taillessness, so that not in the first nor in the second hybrid generation does taillessness appear, and of the Mendelian segregation in the second hybrid generation there is no trace! On the other hand, another cock reveals typical Mendelian phenomena.

Finally, I will speak of a case which is perhaps most instructive of all. I crossed together a double-comb and a singlecomb, and a Polish with a V-comb. The hybrids have a new type of comb called Y-comb; single in front like the stem of a Y and cleft behind like the Polish. Now in the first generation the proportion of the single and the double element varies. At one extreme the single element constitutes 5 per cent.; at the other extreme 95 per cent. Next, I bred together in various generations some of these hybrids with Y-comb birds and, summarizing the results, I found, out of several hundred progeny, about 25 per cent. single comb, 25 per cent. V-comb and 50 per cent. Y-comb. But when I came to examine the families in detail it appeared that whenever both parents had a large proportion of the single in their Y-comb, the proportion of a single-combed offspring rose from 25 per cent. to 40 per cent., or even 50 per cent.; and when the parents had a small proportion of single in their Y-comb, the proportion of single-combed offspring fell from 25 per cent. to 20 per cent. The 25 per cent. of single-comb that one gets on the average seems to prove the Mendelian doctrine of pure germ cells; but the fact that when the tendency to comb in parent is strong, 40 per cent. of the offspring have single-comb; when weak only 20 per cent., throws doubt on the Mendelian doctrine of purity, and hints at a law of relative potency.

In the foregoing I have described the typical Mendelian phenomenon and also various cases of deviation from it — cases of contamination of the so-called pure type extracted from hybrids — cases of lack of dominance, but of particulate or blending inheritance; and cases of potency of one unit character over its antagonist — a potency not constant but varying with the individual or with the strain. Taking all cases into account it is clear that Mendel's law does not cover all; and, if not, it must be a special case of a more inclusive law.

Can we find a more general expression for the inheritance of characteristics which will cover all these cases? I think we can and that it may be called the law of potency. At the one extreme of the series we have equipotent unit characters, so that when they are crossed the offspring show a blend, or a mosaic between them. This is nearly the case with the length of ear in rabbits, as Castle has found; it is probably also true for length of beak in poultry. At the other extreme is allelopotency. One of the two characteristics is completely recessive to the other. This allelopotency is sometimes uniform, as in the case of taillessness in No. 117, heretofore referred to; or in some strawberry hybrids. Usually, it is alternating like taillessness in the son of No. 117. Typical Mendelism comes under the head of alternating allelopotency and it is to Morgan that we owe the first, clear appreciation of the fact. According to his theory, purity of the germ cells does not exist; but a dominant characteristic involves a recessive, as, in electricity, a positive charge involves a negative one. Similarly a recessive characteristic always carries hidden a dominant one. In the first hybrid generation the dominant character is alone evident, but in that generation two kinds of germ cells are produced with opposite characteristics latent. Thus the dominant germ cells have the recessive character latent; the recessive germ cells have the dominant character latent. And segregation is rather functional than structural.

Between the two extremes of equipotency and allelopotency lies the great mass of heritable characteristics which, when opposed in heredity, exhibit varying degrees of potency. This sort of inheritance may he called heteropotency. Heteropotency is exhibited in the case of the elements that make up the Y-comb and in extra toes. It is the usual form of inheritance — that which the breeder relies on when he selects the best appearing animal or plant; or that which throws the largest percentage of favorable offspring.

The law of potency seems thus as a general expression for all the forms and exceptional cases met with in heredity. But it does little to help us toward an understanding of the physical basis of heredity, or towards its control. The next step is to determine the conditions of the different sorts of potency. We know in a general way that progressive characteristics dominate over the more embryonic; but we have to learn the meaning of individual potency and the reason why some pairs of characteristics exhibit equipotency or heteropotency and others allelopotency. As ever in science the formulation of a law only awakens a new series of inquiries.

DISCUSSION.
By W. J. Spillman.

In listening to the address of Doctor Davenport, this evening, it has occurred to me that I might be able to suggest a possible explanation of one of the characteristics noted in the offspring of the hooded fowl. Professor Bateson made many experiments with walnut-combed fowls, interbreeding them with a single-comb, finding the walnut-comb to be compounded of the peacomb and the rose-comb.

It is possible that the "hood" is composed of two characters intead of being a single element in itself.

Suppose the hood character to depend upon two factors T and t, and that T is much more important than t in producing the character. The allelomorphic formula of the pure hooded fowl would then be TT, tt; gametes Tt.

The formula of fowls with plain heads is OO, oo; gametes Oo. The cross is TO, to; gametes Tt, TO, OT, Oo.

Fortuitous union of these gametes gives

1 TT tt 2 TO, tt 1 OO, tt
2 TT, to 4 TO, to 2 OO, to
1 TT, oo 2 TO, OO 1 OO, OO

Thus T and t are both present in 9-
16ths
of
generation
F2.
T alone in 3-
"
"
"
"
t     "    "  1-
"
"
"
"

If T alone is difficult to distinguish from T and t together, this would give twelve hoods and four smooth or nearly smooth heads, or three hoods to one smooth head, which agrees with Dr. Davenport's results. But of the smooth heads three-quarters would show the slight tendency to the hooded condition, due to the presence of character t, provided, of course, my suggestion is correct. This explanation is merely an hypothesis and may or may not be correct.