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HEREDITY

It is a commonplace fact that offspring tend to resemble their parents. So commonplace, indeed, that few stop to wonder at it. No one misunderstands us when we say that such and such a young man is “a chip off the old block,” for that is simply an emphatic way of stating that he resembles one or the other of his parents. The same is true of such familiar expressions as “what’s bred in the bone,” “blood will tell,” and kindred catch phrases. All are but recognitions of the same common fact that offspring exhibit various characteristics similar to those of their progenitors.

Blood Heritage.—To this phenomenon of resemblance in successive generations based on ancestry the term heredity is applied. In man, for instance, there is a marked tendency toward the reappearance in offspring of structures, habits, features, and even personal mannerisms, minute physical defects, and intimate mental peculiarities like those possessed by their parents or more remote forebears. These personal characteristics based on descent from a common source are what we may call the blood heritage of the child to discriminate it from a wholly different kind of inheritance, namely, the passing on from one generation to the next of such material things as personal property or real estate.

Kind Determined by Origin.—It is inheritance in the sense of community of origin that determines whether a given living creature shall be man, beast, bird, fish, or what not. A given individual is human because his ancestors were human. In addition to this stock supply of human qualities he has certain well-marked features which we recognize as characteristics of race. That is, if he is of Anglo-Saxon or Italian or Mongolian parentage, naturally his various qualities will be Anglo-Saxon, Italian, or Mongolian. Still further, he has many distinctive features of mind and body that we recognize as family traits and lastly, his personal characteristics such as designate him to us as Tom, Harry, or James must be added. The latter would include such minutiæ as size and shape of ears, nose or hands; complexion; perhaps even certain defects; voice; color of eyes; and a thousand other particulars. Although we designate these manifold items as individual, they are in reality largely more or less duplicates of similar features that occur in one or the other of his progenitors, features which he would not have in their existing form but for the hereditary relation between him and them.

“O Damsel Dorothy! Dorothy Q.!

Strange is the gift that I owe to you;

····· What if a hundred years ago Those close-shut lips had answered ‘No,’ ····· Should I be I, or would it be One-tenth another, to nine-tenths me?” “Soft is the breath of a maiden’s yes; Not the light gossamer stirs with less; But never a cable that holds so fast Through all the battles of wave and blast, And never an echo of speech or song That lives in the babbling air so long! There were tones in the voice that whispered then You may hear to-day in a hundred men.”

When life steps into the world of matter there comes with it a sort of physical immortality, so to speak; not of the individual, it is true, but of the race. But the important thing to note is that the race is made up, not of a succession of wholly unrelated forms, but a continuation of the same kind of living organisms, and this sameness is due to the actual physical descent of each new individual from a predecessor. In other words, any living organism is the kind of organism it is in virtue of its hereditary relation to its ancestors.

It is part of the biologist’s task to seek a material basis, a continuity of actual substance, for this continuity of life and form between an organism and its offspring. Moreover, inasmuch as the offspring is never precisely similar to its progenitors he must determine also what qualities are susceptible of transmission and in what measure.

Ancestry a Network.—From the fact that each child has all of the ancestors of its mother as well as of its father, arises the great complications which are met with in determining the lineage of an individual. A person has two parents, four grandparents, eight great grandparents, and thus following out pedigree it is plain to be seen that through this process of doubling in each generation, in the course of a few centuries one’s ancestry is apparently enormous. By actual computation, according to Professor D. S. Jordan, if we count thirty generations back to the Norman invasion of England in 1066, at this ratio of duplication, the child of to-day would have had at that time an ancestry of 8,598,094,592 persons. But we know that the total number of inhabitants in England during the time of William the Conqueror was but a small fraction of this enormous aggregate. This means that we shall have to modify our inference that a child has twice as many ancestors as its parents; a condition which at first sight seems evident, but which is not literally true. The fact is that the parents of the child, in all probability, have many ancestors in common—a state of affairs which is brought about through the intermarriage of relatives, and this is especially frequent among remoter descendants of common progenitors. Time after time in genealogy strains of blood have crossed and recrossed until it is not improbable that a man of to-day who is of English origin has the blood in his veins from every inhabitant of England who lived during the time of William the Conqueror and left fruitful descendants. Instead of conceiving of ancestry as an ever branching and widening tree-like system as it recedes into the past, it is more accurate, therefore, to regard it in the light of an elaborate meshwork. The “family tree” in reality becomes the family net.

Ancestry in Royalty.—The pedigrees of royal families have proved to be of much importance in the study of human inheritance, not that royal traits are any more heritable than any other, but simply because the records have been carefully kept so that they are the most comprehensive and easily followed pedigrees available. The netlike weave of ancestry is particularly well exemplified in some of these families because of much close intermarriage. Their heritage typifies on an intensified scale the heritage of the mass of mankind. For example, if we go six generations back in the ancestry of Frederick the Great instead of the expected sixty-four individual ancestors we find only forty; or in a still more closely woven stock, in the Spanish royal line of Don Carlos we find in six generations instead of sixty-four individual ancestors, only twenty-eight. While the present German emperor might have had four thousand ninety-six ancestors in the twelfth generation back, it is estimated that owing to intermarriage he probably had only five hundred thirty-three.

Offspring Derived from One Parent Only.—So far in our reckoning of heredity we have counted elements from both father and mother, and the complications which arise from such a double ancestry are manifestly very perplexing ones. If we could do away with the elements of sex and find offspring that are derived from one parent only, it would seemingly simplify our problem very much for we should thus have a direct line of descent, free from intermingling. This, in fact, occurs to a greater or less extent among lower animals in a number of instances. There may be only female forms for a number of generations and the eggs which they produce develop directly into new individuals. Moreover, many of the simpler organisms have the power of dividing their bodies into two and thus giving rise to two new forms, each of which resembles the parent. This shows plainly that we may have inheritance without the appearance of any male ancestor at all, hence sex is not always a necessary factor in reproduction or heredity. The development of eggs asexually, that is, without uniting first with a male cognate, is termed parthenogenesis. The ordinary plant louse or aphid which is frequently found upon geraniums is a familiar example of an animal which reproduces largely in this way. During the summer only the females exist and they are so astonishingly fertile that one such aphid and her progeny, supposing none dies, will produce one hundred million in the course of five generations. In the last broods of the fall, males and females appear and fertile eggs are produced which lie dormant through the winter to start the cycle of the next year. Again, the eggs of some kinds of animals which normally have to unite with a male germ before they develop, can be made to develop by merely treating them with chemical solutions. The difference between an offspring derived in such a manner, and one which has developed from an egg fertilized by the male is that it is made up of characteristics from only one source, the maternal.

Dual Ancestry an Aid in Studying Heredity.—Although we have the factors of heredity in a more simplified form in the case of asexual transmission, as a matter of fact most of our insight into the problems of heredity has been attained from a study of sexually reproducing forms, because the very existence of two sets of more or less parallel features offers a kind of checking up system by which we can follow a given characteristic.

Reversion.—Occasionally, however, plants and animals do not develop the complete individuality we might expect, but stop short at or re-attain some ancestral stage along the line of descent, and thus come to resemble some progenitor perhaps many generations back of their own time. Thus it is well known that as regards one or more characteristics a child may resemble a grandparent or often some remote ancestor much more closely than it does its immediate parent. The reappearance of such ancestral traits the student of heredity designates as Reversion or Atavism.

Reversion may occur apparently in any class of plants or animals. It is especially pronounced among domesticated forms, which through man’s selection have been produced under more or less artificial conditions. For example, among fancy breeds of pigeons, there may be an occasional return to the old slaty blue color of the ancestral rock-pigeon, with two dark cross-bars on the wings, from which all modern breeds have been derived. This is almost sure to happen if the fancy varieties are inter-crossed for two or three generations. Another example of reversion frequently cited is the occasional reappearance in domestic poultry of the reddish or brownish color pattern of the ancestral jungle-fowl to which, among modern forms, the Indian game seems most nearly related in color. Still another example is the cross-bars or stripes occasionally to be seen on the forelegs of colts, particularly mules, reminiscent of the extinct wild progenitors which were supposedly striped.

Fig. 1, p. 9, is a picture of a hybrid between the common fowl and the guinea-fowl. The chevron-like markings on certain feathers show a reversion to a type of color pattern that is prevalent among both the primitive pheasants (the domestic chicken is a pheasant) and the primitive guinea-fowls. Although the common spotted guinea-fowl may be crossed with a black chicken which shows no trace of barring, nevertheless the hybrid offspring are likely to bear a chevron-like pattern such as that shown in the picture.

There has been much quibbling over the relative meanings of reversion and atavism. The general idea, whichever term we use, is that there is a “throwing back” in a noticeable degree through inheritance to some ancestral condition beyond the immediate parents. A few recent authors have taken the term atavism in a restricted sense and use it to signify specifically those not uncommon cases in which a particular character of an offspring resembles the corresponding character of a grandparent instead of a parent. Such, for example, as the blue eye-color of a child with brown-eyed parents, each of whom in turn has had a blue-eyed parent. The tendency of other authors is to abandon the term entirely because of the diversity of meaning that has been attached to it in the past.

Fig. 1

Hybrid between the guinea-fowl and the common fowl,showing in many feathers reversion to a primitive chevron-like barring.

Certain classes of so-called reversions, such as the case of the eye-color just cited, are readily explicable on Mendelian principles as we shall see in a later chapter, but probably not all kinds of phenomena described as reversion can be so explained. For example, some seem to be cases of suppressed development. The word reversion, indeed, must be looked on as a convenient descriptive term rather than as the name of a single specific condition.

Telegony.—There is yet a wide-spread belief in the supposed influence of an earlier sire on offspring born by the same mother to a later and different sire. This alleged phenomenon is termed telegony. For example, many dog-breeders assert that if a thoroughbred bitch has ever had pups by a mongrel father, her later offspring, although sired by a thoroughbred, will show taints of the former mongrel mating. In such cases the female is believed to be ruined for breeding purposes. Other supposed instances of such influences have been cited among horses, cattle, sheep, pigs, cats, birds, pets of various kinds and even men. The historic case most frequently quoted is that of Lord Morton’s mare which bore a hybrid colt when bred to a quagga, a striped zebra-like animal now extinct. In later years the same mare bore two colts, sired by a black Arabian horse. Both colts showed stripes on the neck and other parts of the body, particularly on the legs. It was inferred that this striping was a sort of after effect of the earlier breeding with the quagga. In recent times, however, Professor Ewart has repeated the experiment a number of times with different mares using a Burchell zebra as the test sire. Although his experiments have been devised so as to conduce in every way possible to telegony his results have been negative. Moreover, it has been pointed out that the stripes on the legs of the two foals alleged to show telegony could not have been derived from the quagga sire for, unlike zebras, quaggas did not have their legs striped. Furthermore it is known that the occurrence of dark brown stripes on the neck, withers and legs of ordinary colts is not uncommon, some cases of which have exhibited more zebra-like markings than those of the colts from Lord Morton’s mare. It seems much more probable, therefore, that the alleged instances are merely cases of ordinary reversion to the striped ancestral color pattern which probably characterized the wild progenitors of the domesticated horse.

Various experiments on guinea-pigs, horses, mice and other forms, especially devised to test out this alleged after-influence of an earlier sire, have all proved negative and the general belief of the biologist to-day is that telegony is a myth.

Prenatal Influences Apart from Heredity.—In discussing the problems of heredity it is necessary to consider also the possibilities of external influences apart from lineage which may affect offspring through either parent. Although modifications derived directly by the parent, and prenatal influences in general, are of extremely doubtful value as of permanent inheritable significance, nevertheless they must be reckoned with in any inventory of a child’s endowment at birth. Impaired vitality on the part of the mother, bad nutrition and physical vicissitudes of various kinds all enter as factors in the birthright of the child, who, moreover, may bear in its veins slumbering poisons from some progenitor who has handed on blood taints not properly attributable to heredity. Of such importance is this kind of influence to the welfare of the immediate child that it will be necessary to discuss it in considerable detail in a later chapter.

Parent Body and Germ Not Identical.—Inasmuch as each new individual appears to arise from material derived from its parent, taking the evidence at its face value one might suppose that any peculiarity of organization called forth in the living substance of the parent would naturally be repeated in the offspring, but a closer study of the developing organism from its first inception to maturity shows this to be probably a wrong conclusion. The parent-body and the reproductive substance contained in that body are by no means identical. It becomes an important question to decide, in fact, how much effect, if any, either permanent or temporary, the parent-body really has on the germ.

A given fertile germ (Fig. 2, p. 13) gives rise by a succession of divisions to a body which we call the individual, but such a germ also gives rise to a series of new germ-cells which reside in that individual, and it is these germ-cells, not something derived from the body, that pass on the determiners of distinguishing features or qualities from generation to generation. It is only by grasping the significance of this fact that we can understand how in certain cases a totally different set of characters may appear in an offspring than those manifested in either parent.

An Hereditary Character Defined.—By a character, in discussions in heredity, is meant simply a trait, feature or other characteristic of an organism. Where we can pick out a single definable characteristic which acts as a unit in heredity, for greater accuracy we term it a unit-character. Many traits are known to be inherited on a unit basis or are capable of being analyzed into factors which are so inherited. These unit-characters are in large measure inherited independently of one another apparently, although cases of characters inherited as a unit along with other characters are known.

Hereditary Mingling a Mosaic Rather Than a Blend.—The independence of unit-characters in inheritance leads us to the important conclusion that the mingling of two lines of ancestry into a new individual is in no sense bringing them into the “melting pot,” as it has been picturesquely expressed, but it is rather to be regarded as the mingling of two mosaics, each particle of which retains its own individuality, and which, even if overshadowed in a given generation, may nevertheless manifest its qualities undimmed in later generations when conditions favorable to its expression transpire.