Chapter 5 - The Chromosomes And Their Genes

We have seen in the chapter on Mendelism that the various traits of plants and animals are carried as unit characters from one generation to another. We know that the determining factors are carried in the gametes (the sperm and the ovum) which unite to form the zygote of the new organism. But what are these determining factors and how are they carried in the gametes to register themselves in the new organism?

We have also seen that each of the billions of cells in the dog's body is made up essentially of transparently clear cytoplasm in which is embodied a tiny gray nucleus. That the cell structure is somewhat more complicated than merely a point of nucleus within the cytoplasm , need not concern us here. It is within the nucleus that we shall find the material which contains the determining factors of the germ plasma .

The nucleus of each cell is a minute, clear, gray mass which contains one or more nucleoli slowly stirring around. As the cell prepares for its division, the nucleoli vanish and are replaced by grayish granules (called chromatin), which in turn draw out into a tangle of fine threads of various length, which writhe and wiggle uneasily. At length these threads slow down their activity. They grow shorter and stubbier as tiny rods of various lengths.

These rods are our chromosomes. The word is derived from the Greek chromo (color), and soma (body). This name is due to the fact that the tiny rods take a deep stain which facilitates their microscopic observation.

The number of these chromosomes in each body cell is constant in each organism and tends to be constant throughout each species. Indeed, some workers consider that the number of chromosomes in each body cell is the final determinant of species, for which word there is no hard and fast definition. (The last statement is not meant to imply that there is a different chromosome count for every single species, but, rather, that the count should be constant within the particular species.)

The reproductive cells before their reduction division contain the same constant number of chromosomes as do the body cells. After reduction division (meiosis), which is to be described somewhat later in this chapter, the chromosome number in the reproductive cells is but half the number in the body cells, and is now called the haploid number.

In Man, the number before reduction division, called the diploid number, is 48; in the mouse it is 40; in Drosophila, the vinegar fly, it is 8; and in the dog it is 78.

In each of the body cells the chromosomes occur in pairs, like the animals in Noah's Ark. The number of the pairs is one-half the total number of chromosomes in any body cell and in the reproductive cells before reduction division. The shape and length of the various pairs may be different but as they are seen under the microscope there is no apparent difference in the members of any single pair, with the exception of the one pair which is known as the sex chromosomes, the so-called X and Y chromosomes, which carry the factor which determines sex, together with other factors.

The sex chromosomes are discussed more fully in Chapter VIII, "The Determination of Sex."

One member of each pair of the chromosomes is derived from each parent, the haploid number (39 in the dog) of chromosomes being in the nucleus of the sperm and the haploid number in the nucleus of the ovum. These unite to provide the full diploid number (78 in the dog) of chromosomes in the zygote.

Now, let us proceed to the process of cell division. There are two kinds of cell division, mitosis and meiosis. We will consider mitosis first. (The reader is urged to consult the diagrams on pages 70 through 74, as we move through this process, phase by phase.)

In Phase I we see the cell at rest, noting its basic structure.

Phase I

There is a cell membrane within which is a large area of cytoplasm . Within the cytoplasm we make out the nuclear membrane enclosing the nucleus, which in this phase is merely a relatively small mass of undifferentiated protoplasm . At the top of the cell is a small, star-shaped body, the centrosome. The cell is at rest and the process of mitosis has not yet begun.

Phase II

The only significant change in this phase is that the undifferentiated protoplasm within the nucleus looks like a tightly coiled mass of thread-like material, the chromatin. The centrosome remains unchanged.

Phase III

The nuclear membrane has dissolved away and the chroma-tin has broken up into a definite number of rods or segments. (Four in our diagram.) These rods are the chromosomes. The centrosome has split into two parts, taking their places at the opposite ends of the diameter of the cell and radiating fine fibers toward the chromosomes. The bodies and their fibers are called the asters. It is believed that the fibers act to pull the chromosomes apart, though the evidence is far from clear on the point. Note especially that the chromosomes are lined up linearly in the cell.

Phase IV

The cell splits down the middle, forming two new daughter cells, each of which is exactly like the original parent cell. The chromosomes have split longitudinally, from end to end. The chromosomes are of the same length but of half the volume.

Phase V

The chromosomes in the new daughter cells regroup themselves in the approximate center of the cell; the nuclear membrane reforms around them; they revert to chromatin and then to undifferentiated protoplasm . The process of mitosis is completed. The daughter cells are then ready to undertake another mitotic division which will be precisely like the one that brought them into being.

Mitosis is a process of cell division in which two new daughter cells are formed from the original parent cell. These daughter cells are exactly like the parent cell and each contains precisely the same number of chromosomes as the parent cell. It is important to remember that in mitosis there is neither an increase nor a decrease in the chromosome number between the daughter cells and the parent cell.

Each of the new cells contain one half of each chromosome which was in the parent cell. These halves function as complete chromosomes in the daughter cells, and when they in turn divide to form four new cells, their chromosomes split themselves lengthwise again, a half of each going into each new cell. Thus we see that the number of chromosomes is constant in all of the somatic, or body, cells.

It is, however, the reproductive cells in which we are particularly interested, and the behavior of the somatic cells is only pertinent insofar as it pertains to them. The chromosomes of the reproductive cells carry within them the factors which determine what the progeny of the organism shall be like, and the animal breeder should know in what manner those factors are conveyed from parent to offspring.

The behavior of the reproductive cells in their division is much like that of the somatic cells, but the reproductive cells undergo reduction division. This process is different in the reproductive cells of the male from those cells of the female.

It should be obvious that if the sperm and ovum should each contribute the full diploid number of chromosomes to the zygote cell of the new organism, each succeeding generation would contain in its cells twice the number of chromosomes of the cells of its parents. With such a doubling of the chromosome count for each new generation, it would not require many generations until all cells contained literally thousands of chromosomes. That, of course, does not happen.

Phase I-Mitosis

Phase II-Mitosis

Phase III-Mitosis

Let us deal first with meiosis (reduction division) as it occurs in the male. We will show it first in diagrammatic form and then discuss it. The reader is asked to compare this process with the diagrams of mitosis.

In meiosis Phases I and II are essentially the same as they are in mitosis. The first noticeable change can be detected in Phase III.

Phase III

The nuclear membrane has dissolved away and the chroma-tin has broken up into the same number of chromosomes as in mitosis. Now, however, the chromosomes do NOT line up linearly in the cell, but, rather, in pairs. Technically, in this formation the chromosomes face each other in homologous pairs, i.e., pair A and Ax are alike in structure; pair B and B1 are alike in structure but are different from A and Ai. In the dog, then, there would be 39 pairs of homologous chromosomes, no two pairs being exactly alike. [Homologous—homo (like) + logos (structure or body)] The action of the centrosome is like that in mitosis.

Phase IV

The cell splits down the middle, forming two new cells, each of which contains only half the original number of chromosomes in the parent cell. This is reduction division. The new cells, not yet functional sperm, contain the haploid count of chromosomes.

Phase V

The two new cells, after going back to Phase I, divide again, but this time the division is a mitotic one so that there is no further reduction of the chromosome count. The four new cells now develop tails and are

functional sperm, i.e., they are capable of fertilizing the ovum. They are ready to go; ready to be used.

In the male, then, meiosis is a process of cell division in which ultimately four functional sperm are formed, each one of which contains only half the original number of chromosomes in the basic sex cell.

In the female the process of meiosis is the same through Phases I, II, III, and IV as it is in the male. In Phase V, however, a distinct change occurs.

Phase V

The two new cells return to Phase I and then divide again mitotically. Four new cells are formed but only ONE of them is a large functional ovum. The other three are relatively small, non-functional polar bodies. In both the ovum and in each of the polar bodies there is the haploid count of chromosomes.

Thus we see that in the female meiosis is a process of cell division in which ultimately one functional ovum and three nonfunctional polar bodies are formed, each one of which contains only half the original number of chromosomes in the basic sex cell.

Two very important factors must be remembered about meiosis. They are:

(1) during meiosis the chromosomes line up in homologous pairs and (2) meiosis is called "reduction division" because when the cell makes its first division the chromosome count is reduced by one-half. (The second splitting of the sex cell, because it is by mitosis, does NOT increase or decrease the chromosome count.

Let us consider the process of making functional sex cells, spermatozoa and ova, in some detail.

In the formation of spermatozoa, the basic reproductive cells divide and re-divide, as in ordinary mitosis, until there are vast numbers of them. Each of these new cells that is to mature undergoes a period of growth in which it not only enlarges but in which the material of its nucleus is much changed. After this period of growth the cell is called a primary spermatocyte, and it is non-functional, i.e., it cannot yet fertilize an ovum.

The chromosomes of these primary spermatocytes gather on the equatorial plane of the spindles as we have seen in the division of the body cells. However, to form sperm they pair off, two and two, each chromosome derived from the organism's paternal parent lining up alongside the homologous one from the maternal parent.

This process of the pairing of the chromosomes preparatory to reduction division is known as synapsis. When the chromosomes have all located their partners, they do not split lengthwise as in mitosis but one member of each pair moves to each pole to form the new cell. In the cells thus formed there are but half the diploid number of chromosomes to be found in a somatic cell, and the reduced number is haploid. Dogs and men are diploid; gametes, functional sex cells, are haploid.

After reduction division, to each cell has gone, in all likelihood, some members of the set of chromosomes derived from the male parent and some members derived from the female parent. It seems to be a merely fortuitous matter, which member of each pair shall go to which pole. It would be remotely possible that all the chromosomes from the sire would go to one pole, all from the dam to the other pole; but the mathematical probability of such a division would be the same as the probability of shuffling a deck with the number of cards of the diploid number of the chromosomes, an equal number of red cards and of black cards, and finding all the cards of either color on top of the pack, all of the other color on the bottom of the pack. Thus, it is all but impossible.

If it should occur, however, the resultant sperm would be related to but a single parent and could transmit to its zygote none of the traits derived from the other parent of the organism in which it arose, and the progeny arising from it in a genetic sense would be quite unrelated to one of its grandparents.

Each of these secondary spermatocytes, as the cells after the reduction division are called, soon divides by the splitting of the chromosomes, as in ordinary mitosis, into two spermatids, each of which sluffs off the largest part of its cytoplasm , grows a long tail, and becomes a spermatozoon, ready to be discharged into the female and to fertilize an ovum.

It will observed that the two haploid sets of chromosomes of the two secondary spermatocytes derived from any primary sper-matocyte must be very different in their content, since those sets are made by the separation of members of homologous pairs rather than by the splitting of the chromosomes of the parent cell.

On the other hand, the two spermatids which result from the splitting of a secondary spermatocyte are exactly alike, for each chromosome in the parent cell has split lengthwise to produce two.

Thus, from each primary spermatocyte comes four spermatozoa, of which there are two pairs which are entirely different, one pair from another, but of which the members of any pair are exactly alike.

Of the billions of spermatozoa proliferated in a single male organism in a normal lifetime, of the millions ejaculated at a single sexual orgasm, of the many thousands in a drop of the seminal fluid, only an almost infinitesimally small number are destined to encounter and fertilize ova; the others are destined to extinction. If a male dog begot two litters of ten puppies each, every week over a period of ten years, he would produce only 14,000 immediate offspring. Such a record has perhaps never been made by any dog, but at a normal single breeding a dog would expel from himself all the sperm necessary and would leave to die disused many, many millions more.

The likelihood that the two like sperm derived from any primary spermatocyte will both succeed in fertilizing ova is so remote that the probability could be expressed only in astronomical figures; but, if they should, the resulting progeny would derive from its male parent a heredity as identical as that of identical twins, produced by the fertilization of a single ovum by a single sperm. The heredity of such progeny derived from the female parent need, however, be no more alike than that of mere fraternal twins, or of two members of a litter. Indeed, fraternal twins are only litter siblings. (The word "siblings," which is not to be found in older dictionaries, denotes the children of the same parents, whether male or/and female. Derived etymologically from the Anglo-Saxon "sib," it is a convenient word and is coming into more frequent usage.)

In the female the basic sex cells, as they develop, undergo a period of enlargement after which they are called primary oocytes, a term analogous in the female to the primary spermato-cytes in the male. The chromosomes of the primary oocyte pair come together in synapsis, as in the male, one set moving to each pole of the spindle. After that, however, instead of the cell dividing into two cells of equal size, there is a most unequal division. The cell throws off a tiny part of itself which contains a haploid set of chromosomes, but very little cytoplasm . This smaller cell is called the first polar body. These two cells, one so much larger than the other, are analogous in the female to the secondary spermatocytes in the male.

The larger cell again subdivides by discharging another cell much smaller than itself, the chromatin content of which results from the splitting of the chromosomes lengthwise, as in ordinary mitosis as we have observed in the splitting of the primary spermatocytes to form the spermatids. This smaller cell is known as the second polar body; the larger one is now the female pronucleus and will soon become a ripe ovum, ready for fertilization by the sperm.

The first polar body, like its larger sister, will again subdivide by the splitting of its chromosomes lengthwise, but the resultant cells and the second polar body cannot be fertilized. Recent evidence indicates that the polar bodies serve as an extra food supply in the event that the ovum is fertilized. However, the polar bodies with the ovum preserve the analogy of the division in the male of the original germ-cell into four cells.
Size in the sperm is a hinderance rather than a help and the spermatozoa sluff off a large part of their cytoplasm in their development—strip for action, as it were. They have no need for any large amount of food material since the nuclear head, with its closely compressed content of chromosomes, is its sole contribution to the zygote. The ovum, on the other hand, must supply most of the material for the development of the embryo while it finds its way through the oviduct into the uterus and there digs in and establishes relations with the maternal organism which will ensure it nourishment for its growth. By stingily throwing off the polar bodies so much smaller than itself, instead of halving and re-halving as does the male germ-cell, the oocyte conserves its cytoplasm for the development of the early stages of the embryo. Some conception of the comparative size of the gametes is conveyed by the estimation that in man the egg, despite its minuteness, is some 85,000 times the volume of the sperm.

We now recognize the chromosomes as tiny, elongated capsules in which are enclosed the stuff of heredity, and we know that they have a constant number for the species and that each of the body cells has the same number. We have seen how that number as it appears in the basic reproductive cell is cut in half to the end that each gamete may contribute the haploid number to form the diploid number of paired chromosomes in the zygote.

But what is inside these capsules which we call chromosomes? The answer is, the genes. And what are the genes? We cannot be absolutely sure. Nobody has ever yet isolated a gene, taken it apart and looked at it. The word gene (Latin for "birth" or "breeding") is used to designate that part of the content of the chromosome which is responsible for any Mendelian factor. For practical purposes, the genes and the Mendelian factors which they convey are one and the same things. Practically, the terms are interchangeable. Technically, the gene is the unit carrier of heredity.

While nobody will yet say definitely that he has seen a gene, Painter, of the University of Texas, and Bridges, an associate of Morgan at the California Institute of Technology, have seen and made microphotographs of minute bands along the chromosomes of the large cells of the salivary glands of Drosophila. These bands, they believe, may be the genes.

While it is not known exactly how a gene is chemically constituted, it is known that genes lie in a systematized order along the length of the chromosome, like beads on a string. In any homologous pair of chromosomes in a given species, the gene for any specific factor exists in the same part of one member of the pair as the gene for the same factor or its allelomorph exists in the other member of the same pair. (For the distinction between alleles and allelomorph, the reader is referred to the glossary.)

In the above pair of homologous chromosomes, genes A and Ai are alleles; genes B and Bx are alleles, genes C and C1 are alleles. Thus we note that the chromosomes are homologous not only in their external structure but in the gene structures as well.

In each haploid set of chromosomes, the gene for any given factor or its allelomorph is to be found in the same chromosome; and, not only the same chromosome, but in the same position in relation to the other genes along the length of the chromosome, just as in the spine the first lumbar vertebra has a constant position between the twelfth or last dorsal and the second lumbar vertebra. So a given gene or its allelomorph has a given normal position in the arrangement of the genes along the chromosome. Abnormalities in the behavior, of the chromosomes do occur, and they are likely to cause either the death of the zygote or the malformation of the organism which derives from it. For the most part, however, the chromosomes can be depended upon to keep their regimented place.

The work of Thomas Hunt Morgan with Drosophila has contributed more to the knowledge of chromosomes and their behavior than has that of any other single worker, winning for him the Nobel Prize in Medicine in 1933.

Morgan was able to formulate a map, not yet and perhaps never to be complete, showing the arrangement of several hundred of the various genes along the various chromosomes of his beloved vinegar flies. Morgan's map was made before Painter and Bridges had observed the bands on the chromosomes which are believed to be the genes. This accomplishment is staggering in its seemingly insuperable difficulties. That Morgan has been able to declare that any recognized gene, let us say the gene for red eyes or its allelomorphic gene for white eyes, lies in a given chromosome and in a definite position in relation to the other given recognized genes in that chromosome, is due to the comparatively small number (diploid, eight; haploid, four) of the chromosomes in the Drosophila cell, to the rapidity with which it reproduces itself, with a new generation every ten days, and to the small size of the flies, which enables one worker to maintain vast numbers of them. Even then, but for the phenomenon of "crossing-over," the exchange of places of two or more genes in one chromosome with the same gene or its allelomorph in the other member of the chromosome pair, this mapping would have been impossible.

We return here briefly to the consideration that the Second Mendelian Law, that the sets of alleles behave independently one of the others, is not categorically true. The phenomenon that saves the Second Law from at least partial invalidation is "crossing-over."

Mendel was not aware of the chromosomes and their genes. He found that the sets of factors behaved in a certain fashion in the garden peas, but he did not know the physical mechanism of such behavior. He observed the phenomena without being able to understand their causation.

The factors with which he worked are in the garden pea each dependent upon a single set of allelic genes. And the sets of alleles in the garden pea are to be found in different pairs of chromosomes. It was only the genetic simplicity of the garden pea, as pertains to the sets of factors with which he experimented, that enabled Mendel to reach his conclusions. When he later worked with hawkweed and other plants he was baffled by the failure of what he thought to be simple factors to behave as they had done in peas. We know now that his bafflement was due to the circumstance that some of the characteristics of a plant which he thought depended upon a single factor, in reality depended upon two or more factors, i.e., two or more allelic pairs of genes.

We may amend the Second Mendelian Law to say that the sets of genes within any pair of chromosomes behave independently of the sets of genes within any other pair of chromosomes. While it is indeed true that the genes are the unit carriers of heredity, they are located within specific chromosomes and it is the chromosomes that behave independently.

This tendency of particular genes to remain together within a particular chromosome and to be passed on together is called linkage. If Mendel had been aware of the phenomenon of linkage, he would not, perhaps, have promulgated the Second Law precisely as he did. If the factors for smooth seed and for tall growth in the garden pea had been in one chromosome, instead of in separate chromosomes as they are, Mendel would not have been able (barring the possibility of "crossing-over") to breed tall peas with wrinkled seed nor dwarf peas with smooth seed, and could not have declared the independent assortment of the genes.

We have said that there is a "tendency" for given genes to stay in their own chromo somatic backyard. Adherence to such tendency is the normal behavior. But the violation of that tendency does take place, and it is this observance of behavior that saves the Second Mendelian Law.

When the chromosomes pair off for reduction from the diploid to the haploid number, one or more genes of one chromosome may exchange places with the homologous genes of the other member of the pair of chromosomes. (See diagram on p. 76.) It appears that in synapses, the members of a pair of chromosomes sometimes become entangled one with the other, and one or both ends of both break off and exchange positions, joining on to the correct place in the opposite member of the pair. This occurs at either end of the chromosomes and the fragment may be of a single gene or any number of genes. If it occurs at both ends of the same pair in one synapsis, it is as if a segment from the middles were exchanged. This phenomenon is known as "crossing-over."

It is as if, in dancing a Virginia reel, a part of the pairs of partners at the end of a line should exchange lines, each making the exchange with his own partner. This might occur at both ends of the line, leaving one or more pairs of partners in the same line as before.

From the observations of thousands of individual Drosophila, it was possible to determine which genes tended to linkage within which chromosome. Having located the genes in their respective chromosomes, and recognizing that "crossing-over" occurs at each end of the chromosome, it is reasoned that the gene which is observed to be most frequently out of its normal place is the end gene of its chromosome, that the gene from the same chromosome which shifts with the second degree of frequency is the second from the end; and so the map of the four haploid chromosomes of Drosophila was drawn.

With such a map and an assortment of Drosophila of known ancestry, it is possible to produce at will flies with any recognized assortment of traits.

To make such a map of the chromosomes of the dog by such a method would involve a thousand years of time and the maintenance of a kennel as large as the harem of Solomon. The large number of chromosomes involved, the length of time required for each new generation, and the great amount of space, food, and care necessary for so large a number of dogs would place the undertaking quite beyond the boundary of the practicable. Indeed, the possession of such a map, finished and complete, with an adequate assortment of dogs, would reduce the breeding of dogs from a creative art to an exact science, even to a mere assembly of traits like the putting together of standardized parts of a machine. The breeding of dogs would be just about as interesting then as erecting pre-fabricated houses. And the results would be as stereotyped.

Not only do the genes "cross-over" from time to time, but occasionally chromosomes break up into permanent segments, or one chromosome may affix itself to the end of another, pr a part break off of one and join another. Such behavior is rare and abnormal and is of little practical interest to us. A zygote formed of the union of such a gamete cell with a normal one, if it survives at all, would probably produce an abnormal organism fit only for destruction. However, it is possible for a variety with new and desirable variations so to arise. The breeder of dogs may, however, well ignore such a possibility. It is recognized that change of the genetic contents of a cell may be effected by X-radiation, by excessively high heat, or by certain chemicals, such as colchicines; but until further research yields controlled effects such changes can be of no value in the betterment of the higher animals.

There is no danger that we shall ever learn enough of the mechanics of the heredity of the dog, however, to sink in the prosaic certainty of a standardized product the fascination which attaches to dog breeding. An absolute rule-of-thumb procedure would destroy at once the art and the pleasure. However, in our yet benighted ignorance of the subject, lighted only faintly by the morning star of Mendelism and the nascent dawn of recent work in genetics, any new knowledge which may guide us along the way to better dogs cannot be ignored. Substitute exact knowledge as far as we are able for the haphazard of trial and error, we yet have left enough of uncertainty, and will have for centuries to come, to give zest to the breeder's art.

The value to the dog breeder of the knowledge of the chromosomes and genes is that it brings to him a realization that when he brings together two dogs of opposite sex for the purpose of producing a litter of puppies, he is not merely mating female to male, bitch to dog, egg to sperm, or even chromosome to chromosome, but rather he is pairing two hosts of genes. What the puppy shall look like, how he shall behave, how he shall thrive in a given environment, and even how long he shall live depend upon the genes which find their way to the zygote.

We thus see that the age-old idea of the mingling in the progeny of the blood of the two parents is a fallacy. The individual is a mosaic of the hereditary factors of the two ancestral lines, rather than a blend of the parental bloods which in their turns had been blended from the bloods of their ancestors.

From such a mosaic, the creative breeder can choose the genetic stones he wishes for the design of the ensuing generation, whereas, if we were dealing with blended blood it would be impossible to segregate the desired part from the undesired part of the contents. We are dealing, to use a figure of speech, with platters full of hard boiled eggs rather than of scrambled eggs, and it is much easier to sort out and discard the rotten eggs from the hard boiled units than to unscramble the eggs and get rid of the unfit parts of such a mass.

To change our figure again, the chromosomes and their genes as they appear in the zygote are a full set of plans and specifications for the organism. Therein every detail is laid down and described minutely. If environment, as the executive builder from those chromo somatic and genetic plans, fails of its execution, it is no fault of the breeder-architect who made the blueprints.

The chromosomes and their genes have been likened in heredity to the molecules and atoms in chemistry; and if it were possible to analyze an organism's genetic content in the same manner in which matter's atomic content can be analyzed, we should be able to produce in our dogs whatever traits we chose.

Without the map of the genes within the chromosomes, which we shall perhaps never achieve for so complex an organism as the dog, we shall not be able to breed exactly the kind of dogs we wish every time we try. But the chromosomes and genes are the material elements of Mendelism and understanding of them leads to profound implications which have their practical application in the breeding of dogs. These "Implications of Mendelism" will form the subject matter for Chapter VII.

Many facts about the chromosomes and genes of only theoretic interest to the breeder have, for lack of space and for their impracticability of application to the subject in hand, been omitted here. The subject must needs be intriguing to anybody enough interested in the breeding of dogs to attempt the comprehension of the apparent miracles here described, and even the host of facts and theories as they pertain to other organisms than the dog are worthy of the dog man's interest. The brief bibliography attached to the volume will make possible to such readers as may be curious the exploration of the byways of the biology of reproduction.

Indeed, to many readers it may seem a far cry from garden peas to Great Danes, or from vinegar flies to Fox Terriers. However, garden peas and vinegar flies have served as tiny candles (to glimmer only dimly, it is true) in the darkness where the dog breeder has been working. That early breeders achieved our beautiful varieties of dogs with the scant knowledge at their command is a credit to their patience and persistence. With our present and increasing knowledge, if the dog fancy will utilize it, we can improve those varieties with a celerity and certainty undreamed of by the enthusiasts who founded them and brought them as far as they have come. A sound knowledge of the materials of heredity, of the genes and chromosomes and their behavior will save us from a vast number of false starts and a vast amount of wasted effort. If it does nothing else, it will prune a lot of dead wood, delete a host of old-wives' tales and superstitions from previously entertained theories of breeding. It shows us why these "Things That Are Not True," as we call them in Chapter XIV, cannot be true, and enables us, in disregarding them, to divert the efforts otherwise spent upon them into the channels of constructive breeding.

Upon the person who wishes simply to produce some puppies eligible to registration, for the purpose of sale or of the gratification of some vanity in winning a few cups and ribbons, the mysteries of genetics are wasted. Why should one bother about so small a thing as a gene? Nobody knows what a gene is anyway. His bitch has won a ribbon and he is going to mate her to the latest and most fashionable champion and trust to luck. May he have his luck, with our blessing! Such a person is not a dog breeder in our sense that the breeding of dogs is a creative art.

The true breeder of dogs is one interested in the betterment of dogs for dogs' own sake; interested, not in producing a single chance specimen of some degree of excellence, but rather in the improvement of the whole breed as much by conserving the good in the germ plasma s of the strain as by eliminating the bad; interested not in a single generation, but in the breed's whole future. To such a person (certainly in the long run and probably in the beginning) will come more of monetary profit and of consistent show-ring success than the chance trifler with dogs can ever attain.

And no consideration can be of greater value to the serious breeder of dogs than the realization that his dogs and bitches, from a genetic standpoint, are merely bundles of genes neatly wrapped up in chromosomes. It is his business to deal those decks, of which the chromosomes are the cards and the genes are the pips on the cards. If he can mark and stack those cards in his little game with old Dame Nature, who has herself often enough taken unfair advantage of the breeder's ignorance, it but evens the score. Hitherto he has mixed the drinks of blood and blood, permitting the old lady to shuffle the cards of chromosomes and genes. Try as he may to cheat her, she will out-cheat him and win often enough.

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