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Farm Animals - The Hereditary Material

( Originally Published 1912 )

Although stock breeders have received many interesting suggestions and directions from scientists during the last few years the facts of heredity are far from being an open book even to the scientists. It may be stated here that the most suggestive expressions the scientists have given to the breeders relate to the creation of new forms and the production and fixation in our domestic animals of characters not now common. In these pages the endeavor is chiefly to pre-sent such matter as may permit a more complete under-standing of the physical basis of breeding, and the object is not so much the discussion of means of adding new types and characters as it is to stimulate a study that shall result in greater uniformity of excellence among the existing stock and a closer resemblance of the majority to the present best. In arriving at the fertilization of the ovum by the spermatozoon, or the planting of the seed of the new animal, we have adhered to incontrovertible facts. Though entirely probable it has not been fully demonstrated that the chromatin is the exclusive seat of heredity. Even if it is not it is the active part of the cells which do carry all of heredity, and its changes are highly significant.

So far as concerns any intimate knowledge of the make-up of the chromosomes or the distribution among them of the control of various portions of the body we are entirely in the dark. We may at best recognize the combined chromosomes as carrying all that is transmitted and any practical consideration thereof must regard the chromosomes simply as portions of the hereditary material. Whether one chromosome could by itself if necessary direct the development of an entire animal or whether the germs of different parts or organs are carried in separate chromosomes can hardly be conjectured. Regarding the chromatin simply as the hereditary material, with the facts that have been stated be can account for the lack of similarity in the offspring of two parents. Farmers who undertake to raise a pair of matched horses by breeding a mare to the same stallion in two successive seasons are frequently at a loss to account for the great disappointment.

Every cell in the body contains sixteen chromosomes, the direct product of the original group bequeathed equally by the two parents. In the preparation of the germ cells we know that one-half the chromatin bodies are eliminated. Considering at present, for the sake of clearness, a female of a species for which four is the regular number of chromosomes, we know that in a germ cell of that female only two chromosomes will be present to convey hereditary influences. It is impossible to know or foretell which two of the original four bodies will be preserved. If we consider the four chromosomes to bear numbers from one to four, then while one and two may be in the ovum produced at one period, another ovum produced may contain the same combination again, or, it may contain numbers three and four or any two of the number present in whichever one of the primitive egg cells is being developed into an ovum. Any one of the following combinations is equally as likely as any other to be present in the ovum produced at any certain period :

1 and 2,
1 and 3,
1 and 4,
2 and 3,
2 and 4,
3 and 4.

That the dam of an animal of a species of even four chromosomes should make exactly the same contribution to two successive offsprings is highly improbable, yet our larger animals have sixteen chromosomes to a cell.

The same considerations obtain on the sire's side. All divisions of the primitive spermatozoa remain functional, but only one is utilized in fertilization, therefore the probabilities are the same as with the female. Designating the chromosomes of the paternal cells as five, six, seven, and eight, a single spermatozoon has equal chances for carrying any one of the following :

5 and 6, 6 and 7,
5 and 7, 6 and 8,
5 and 8, 7 and 8.

When fertilization occurs we know that some one of the equally probable maternal combinations will unite with some one of the equally probable paternal combinations. Any one of the pairs in the ma ternal list has equal probability of joining with any one of the paternal list.

Maternal Paternal

1 and 2, 5 and 6,

1 and 3, 5 and 7,

1 and 4, 5 and 8,

2 and 3, 6 and 7,

2 and 4, 6 and 8,

3 and 4; 7 and 8.

The offspring will receive one of these equally probable sets of chromosomes :

1-2 and 5-6, 2-3 and 5-6,

1-2 and 5-7, 2-3 and 5-7,

1-2 and 5-8, 2-3 and 5-8,

1-2 and 6-7, 2-3 and 6-7,

1-2 and 6-8, 2-3 and 6-8,

1-2 and 7-8, 2-3 and 6-8,

1-3 and 5-6, 2-4 and 5-6,

1-3 and 5-7, 2-4 and 5-7,

1-3 and 5-8, 2-4 and 5-8,

1-3 and 6-7, 2-4 and 6-7,

1-3 and 6-8, 2-4 and 6-8,

1-3 and 7-8, 2-4 and 7-8,

1-4 and 5-6, 3-4 and 5-6,

1-4 and 5-7, 3-4 and 5-7,

1-4 and 5-8, 3-4 and 5-8,

1-4 and 6-7, 3-4 and 6-7,

1-4 and 6-8 3-4 and 6-8,

1-4 and 7-8, 3-4 and 7-8,

It is apparent that from two parents of a species with four chromosomes it is possible to have thirty-six individuals, no two of which would be identical. Of course, the majority would largely be of the same make-up, but in the two combinations written first and last there would be a very wide dissimilarity. The contrast of these two possibilities is the basis of the seeming impossible stateent we sometimes hear, that two offspring of the same parents may be unrelated to each other.

If we consider our larger animals that are believed to possess sixteen chromosomes we find amazing possibilities. Taking the possible number of combinations of eight chromosomes that can be made up from sixteen in either parent and exhausting the number of unions that may be produced from these two sets, it is found that without duplication we may have combinations to the number of 65,536.

In view of the immense field of possibilities it is not surprising that. we seldom find two animals that even seem to be identical or even nearly enough so to make a matched pair.

Some of the high-class animals produced by supposedly indifferent parents are doubtless the outcome of the rare occurrence of a combination of the best of material of each parent and the elimination of that tending to produce inferiority. Also some of the very mediocre offspring of renowned parents may be attributed to an exceedingly unfortuitous retention of the chromatin productive of inferior characters and the elimination of the desirable. It is undeniable that in this vital process of heredity there is and must ever be a large element of chance. Chance may govern what portions of the material will go to each offspring, but if it were possible to assure ourselves that all of each parent's supply was representative of good we might be careless of chance. However, it is not necessary or justifiable to assume that each chromosome is entirely different from all the others in the same or in another parent. In all probability they are largely similar. But they may be arranged in an infinite variety of ways and this arrangement is beyond all control. Though impossible to bring any influence to bear upon the manner of separating the portion of hereditary material for the new animal, we can yet assure ourselves of a desirable outcome by limiting ourselves to such animals as give us reason to believe that any selection from their stock of hereditary substance would contain the minimum of possibilities for undesirable characters.

To achieve the greatest possible measure of control over heredity is the aim and need of the breeder of animals adapted to special uses. He redity is chiefly if not entirely conveyed by the chromosomes of the germ cells. The elimination of some of these chromosomes and the amazing array of combinations that may be effected have all to do with determining the make-up of every creature. No degree of human influence over these processes is conceivable. How then has it been possible for the builders of breeds and types to mold the animal form so nearly to their liking? The answer is, by the selecting for mating of animals containing chromatin or hereditary material with the maxi-mum possibilities of desirable features and the minimum of those undesirable; this done, no matter what hereditary bodies are eliminated or combined the result is still for good, and any few chance representations of unwelcome qualities are hopelessly in the minority.

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