Animal Breeding Heredity of Normal, Abnormal and Acquired Characters
Animal Breeding Heredity of Normal, Abnormal and Acquired Characters All transmission is the outcome of natural law. But it has been shown that the laws which govern transmission are not equally apparent. For instance, the laws that control variation and reversion are more obscure in their action than the laws that control likeness in transmission.
The same things may be said of the heredity of normal, abnormal, and acquired characters. While all such heredity is under the control of law the transmission of normal, abnormal, and acquired characters is by no means equally uniform or equally apparent.
Definition of Heredity. — Heredity is the result of the operation of that law whereby characters and qualities of like kind with those of the parents and ancestors are transmitted to the offspring. It is another name for inheritance and is so closely akin to transmission that the terms as applied to breeding may be considered synonymous and interchangeable.
This transmission relates to structure, function, habit, and qualities, and, indeed, to every feature of the organization. Thus far it is on a par with transmission which is the outcome of the law that like pro- duces like. But, unlike the former, it relates to all kinds of transmission as well as to the transmission of like qualities. The inheritance of spavin in a colt from one or both parents thus affected illustrates- heredity that relates to structure.
The inheritance of high stepping and free knee action in the progeny of one or both parents thus characterized illustrates heredity relating to function. The difficulty found in teaching a calf to drink milk when immediately de- scended from ancestry that have roamed on the range, as previously stated, as compared with a Holstein calf whose ancestry have been reared by hand for generations, illustrates heredity which relates to habit.
And the superior handling qualities in an Aberdeen Angus Poll descended from parents that were thus furnished, as also previously stated, illustrates the heredity of a quality, that is to say, the quality of good handling. The supposed exceptions to heredity are doubtless the result of the predominant influence for the time being of other laws acting in opposition to the hereditary tendency. Heredity may be -characterized as normal, abnormal, and acquired. These will be considered separately.
Heredity that is Normal. —By the heredity of normal characters is meant the inheritance or trans- mission of characters natural to the type. These characters are of two classes. They have been original traits bestowed upon the species, or they may have been acquired, and rendered permanent by continued transmission.
To the former class may be referred the readiness with which the horse obeys, the teachableness of the dog, the natural timidity of the sheep, the thirst of the tiger for blood, the readiness with which swine seek the wallow in time of heat, and the eagerness of the collie dog to assist in driving the flock without harming it. It is not always possible to distinguish between heredity that is normal to the type and heredity that is acquired.
For instance, the habit of milk giving in the cow is normal, but the habit of abundant milk giving is acquired. That which is acquired may in time come to be looked upon as normal when it becomes so engrafted on the species as to be transmitted with as much regularity as characters that were original traits.
Illustrations of Normal Heredity. —Illustrations of the persistent and uniform action of the law of heredity of normal characters may be readily drawn from the different departments of organic life. In geological formations covering immense periods of time, fossil species and generic forms present the same essential characters throughout the entire range.
The ox, for instance, has the essential features of anatomical structure in his organization in the skeletons found in the earliest geological formations, as he possesses it to-day. The lapse of time represented in the historic period has made no appreciable change in the characters of wild animals. The lion is neither more nor less fierce to-day than in the days when he was hunted by Ximrod.
The wild hare is no less timid than it was long centuries ago. The elephant of today is characterized by its enormous size as he was in the days when he trod down the enemy in battle while Alexander the Great was conquering Persia.
The wild goat loves to graze upon the mountains as in the days before the flood. And the eagle builds its eyrie in the cleft of the rock as it did in the days when the earth was green and young.
The animals that have been preserved in the monuments of Egypt from a remote antiquity are the same as those now found on the banks of the Nile.
In these various illustrations is evidenced the constancy and persistency with which original traits are transmitted when nature is not interfered with in her processes.
It would, therefore, be correct to assume, that unaided nature has not perform^ any important part in the evolution of breeds. It is only, or at least chiefly, when the guiding hand of man has come to the aid of nature that evolution has become at all permanent.
Heredity of Individual Peculiarities.— The heredity of normal characters is by no means confined to those that belong to the species as such. It extends as well to individual peculiarities. Illustrations of such transmission are found, first, in the families of athletes and giants. In the Old Testament Scriptures families of giants are referred to and in a way which points to the conclusion that such extraordinary physical development was the outcome of inheritance.
In ancient Greece were families of athletes noted as such from generation to generation. Second, in the very large number of running horses descended from Herod and Eclipse and of trotting horses descended from Messenger. The winners in the individual progeny of these horses aggregate a substantial number.
Third, in the remarkable development of the musical talent found in certain families for successive generations. As Miles has stated, many of the descendants of Sebastian Bach, who lived in the sixteenth century, were organists and church singers in Thuringia, Saxony, and Franconia.
For two centuries subsequently to his death they furnished many musicians of great eminence. Fourth, in the longevity of certain families even though the conditions are unfavorable to such longevity.
To so great an extent is longevity in families taken as the estimate of the duration of life, that insurance agents lay much stress upon it when issuing policies. Fifth, in the fecundity or sterility of certain families.
This is witnessed in humans and also in domestic animals, and these distinctions are noticeable where the conditions of life are terribly similar. And sixth, in the inheritance of mental and moral traits. Some families are noted as being quarrelsome from generation to generation, others are pre-eminently distinguished by their generosity, and yet again others are noted for the protracted line of individuals who have devoted their lives to the Christian ministry.
Heredity that is Abnormal. —The heredity of abnormal characters means the inheritance of characters which have deviated from the natural and ac- quired characteristics of the type. It would, perhaps, be correct to say that all diseases are abnormal, and that the discussion of the same should be included in the discussion of abnormal characters. But there are some points of difference between disease and other abnormal characters.
While all diseases are in a sense abnormal, characters that are usually spoken of as abnormal do not necessarily constitute disease. For instance, there may be malformation of structure, as an excess in the number of fingers, or derangement of function, as deafness, found in persons who have excellent health.
There are more obscure surrounding abnormal characters which do not constitute disease than in those which do constitute the same. It has, therefore, been deemed advisable to discuss the two separately. Abnormal characters which do not constitute disease may appear as malformations of structure or derangement of function. When they constitute disease, they assume many forms of the same as will be shown in Chapter VII., which treats of the heredity of diseases.
Heredity of Malformations in Structure. —The heredity of malformations in structure may be illustrated as follows:
- Certain families have been found with an excess or with a deficiency in the number of fingers and toes or of joints in the same, and these have been inherited with more or less regularity for generations.
Females in the human family have been able to give nurse to their children from more than two nipples, and some of these have been irregularly placed.
- Dorking and Houdan fowls have a fifth toe, which is, of course, supernumerary. The Houdans especially were not always characterized by this peculiarity, though now it is a constant characteristic of the breed, and in the Dorkings the tendency is strong to further variation in the pro- duction of toes.
- The Niata breed of cattle grazed by the river Plata. As described by Darwin in “Animals and Plants under Domestication,” they had a peculiar malformation of the skull, by which its nasal end was curved upward. The lower jaw projected beyond the upper and had also a corresponding upward curvature. It is reasonable to suppose that this deformity is the outcome of inheritance in malformation.
- A family of one eared rabbit has been originated, as described by Dr. Anderson in “Recreations in Agriculture,” by breeding together a closely related pair in which the abnormal character had appeared. Illustrations of this feature of ab- normal transmission could be multiplied indefinitely.
Heredity of the Derangement of Function. — Illustrations of the derangement of function may be found in the tendency of some families to use the left hand. This tendency has frequently been transmitted from generation to generation. The tribe of Benjamin was noted for its left-handed slingers. These appear to have been so numerous that they constituted the rule rather than the exception.
The narrative would seem to show, at least, that the most famous of the slingers in the tribe were left-handed. Further illustration of the same is found in the inheritance of deafness, dumbness, and impaired vision.
These are frequently transmitted from generation to generation though not by any means in an unbroken line of transmission. The inheritance may be direct and obvious in some members of a family, but not so obvious in others, and yet it may reappear in the children or in the descendants of the latter.
Abnormal Characters not Uniformly Inherited, —Abnormal characters are not so likely to be perpetuated through transmission as original traits or acquired habits in harmony with the original peculiarities of the animal. If a record were kept of all the instances of inheritance of abnormal qualities in the offspring thus affected, it would be found that as a rule the number of the progeny not inheriting such qualities would exceed that of the progeny in which such inheritance appeared.
But this result may arise in part from the fact that parents possessing abnormal peculiarities are usually mated with those who do not possess them. Normal inheritance from the latter would tend to counteract ab- normal inheritance from the former.
But, sometimes, there is an increase in the development of abnormal characters. Such would appear to be true of Houdan fowls, which, according to Wright in his work on poultry, very rarely showed the fifth toe when first introduced into England, while now the absence of the fifth toe is exceptional. Why the tendency to transmit abnormal characters should be so much stronger in some instances than in others cannot be fully accounted for in the present state of our knowledge.
Abnormal Transmission not always Apparent. — It is not by any means certain that the abnormal peculiarities of parents have not been transmitted to the offspring when they are not discernible. Sometimes such transmission is not apparent for at least a limited number of generations, when suddenly it reappears with more or less completeness.
The fact of such reappearance as with ordinary atavic transmission proves that the tendency to these abnormal characters has been transmitted all the while. Their obscurity in the meantime has been the result of the presence of some more dominant character or characters. Their non-inheritance can only be fully determined by an exhaustive examination of all the individuals in the direct and collateral lines of descent, and for a period that will cover several generations.
Functional Derangement not Always Followed by Structural Changes.— The transmission of functional peculiarities does not always involve the transmission of some corresponding structural change. Functional derangements from an injured nervous system have frequently been transmitted without malformation of the nerves. Nor does the inheritance of the use of the left hand involve any structural change. In a majority of instances, it would be correct to say that such transmission is accompanied by structural changes.
For instance, the inheritance of diminished capacity for milk production, as when the inheritance comes from the male, is accompanied by less capacity in the development of the udder, and by an udder less glandular in character. It should also be remembered that in-and-in breeding tends to intensify all forms of abnormal inheritance. It should, therefore, be most sedulously avoided when the aim is to breed out abnormal characters that may have appeared.
The Heredity of Acquired Characters. —By the heredity of acquired characters is meant the inheritance of characters engrafted upon those original traits peculiar to the type. They differ from normal characters in not having originally belonged to the type, and they differ from abnormal characters in their being in harmony with the original constitution of the race, which the latter are not. They may be produced by such influences as food, environment, education, and training.
The much greater size of the American Merino as compared with the Spanish Merino is an acquired character produced by good food aided by good care. The relatively large development of the forequarters of the sheep reared on mountains is an acquired character produced by environment.
The readiness with which domestic animals submit to human direction as compared with wild animals of the same species is a characteristic acquired through education. And the tendency in the collie dog, as previously intimated, to drive at the heels rather than at the head is a character acquired through generations of training. These characters may be more quickly secured and intensified by the aid of in-and-in breeding than in its absence. Especially is this true when they are in the formative stage.
Heredity of Acquired Characters Illustrated. — Illustrations of the inheritance of acquired characters are numerous :
1 . It is seen in the sagacity and fidelity of the collie dog and in the striking peculiarities of other breeds of dogs. The wisdom of the collie dog’ is such that it would almost seem to be guided by reason. The readiness with which the Newfoundland dog takes to the water is simply wonderful. So, too, is the service rendered by the St. Bernard dogs of the Swiss Alps, and by the pointer and the setter. These distinguishing traits are all acquired.
- It is seen in the tendency of beef breeds to lay on fat and of the dairy breeds to secrete an abundant supply of milk and for an extended period. The marked differences which in these respects characterize breeds did not always characterize them. They have been first acquired and then intensified.
- It is seen in the speed of the American trotting horses, in the ambling pace of those of the Cordilleras, and in the readiness with which Norwegian ponies obey the human voice. These characters have been developed through long years of training, until they have come to be transmitted with much regularity.
- And it is seen in the disposition to wariness which has come to characterize various races of Avild animals, which at one time manifested no uneasiness because of man’s presence.
Birds, and also quadrupeds, inhabiting various islands when these Avere first discovered, manifested no fear of man, but their descendants now flee at his approach.
Acquired Characters and Original Traits Conflicting Elements.— From what has been said it will be apparent that acquired characters and original traits are conflicting’ elements, either one of which, from its intensity, may predominate in hereditary transmission.
The former is less certain to appear for a time in heredity, but eventually they may be looked for with as much certainty as the original traits. In many instances they supplant the former. Such was the case with the wild birds just referred to in which the absence of fear because of man’s presence was succeeded by great fear because of the same. From a practical point of view, therefore, the engrafting of acquired characters is without any limit.
The fact is not to be lost sight of, however, that the characters thus engrafted must be in harmony with the original constitution of the race. The dog is naturally teachable, hence, with the dog the engrafting of an acquired character is not usually difficult. Swine are not nearly so teachable by nature. Generations of careful training would doubtless improve them in this respect but would never make them so susceptible to training as the dog.
It is not difficult to modify physical characters in animals that were of primal bestowment, that in turn they become acquired characters. The strong tail head possessed by many of the old time Galloways is much less pronounced in the Galloways of today.
The reduced size of the tail head has, therefore, become an acquired physical character of the breed. But it is difficult to eliminate an organ of primal bestowment, such as, for instance, the removal of horns or tail in domestic animals without the help of spontaneous variation.
During past centuries it has been customary with some, at least, of the English breeds of sheep, to remove the tail at an early age, except for from one to two inches nearest the root of the same, and yet when lambs of these breeds are born, the tails are as long as they ever were. The change would be made much more quickly by constantly selecting animals for breeding possessed of the shortest tails. It would, therefore, require many generations of breeding be- fore dehorning alone would produce hornless animals, if, indeed, that alone would ever produce such a result.
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