Chapter 8 - The Determination Of Sex

Breeders of dogs would like to have a method by which they could so influence the sex of the puppies whelped by their bitches that they should be able to obtain within the litter a preponderance or a totality of the sex preferred. Such a method has not yet been developed. It is entirely within the realm of possibility that at some future date animal breeders will be able to produce only males or only females, as they choose, from any mating. That time is not yet.

Various methods of influencing sex are recommended; none so far developed is of any practical worth. They are in their essence superstitions and old-wives' tales. One or another of them may have within it a germ of truth, which may in time lead to a means of producing puppies of the sex we may want.

Among the methods suggested is the one of breeding the bitch early in the period of heat to produce female progeny, late in her heat to produce male progeny. Such a method is efficient just about fifty percent of the time, which only means that it does not work at all since approximately half the dogs born are of one sex and half of the other, even without any effort to influence their sex. This method, or perhaps its reverse, was recommended by so intelligent an author as Axtell in his The Boston Terrier, which appeared about a quarter century ago, along with much other now discredited hocus-pocus.

And yet by forcing a female frog to retain her eggs for three or four days after they are fully ripe, it has been possible to produce from them when fertilized some ninety to one hundred percent of males instead of the usual approximately fifty percent. This might by analogy lead to credence that late breeding tends to produce males.

It may be that very ripe or over-ripe ova are more penetrable by the male producing sperm than are less ripe ones. Out of such a hypothesis might possibly grow a method of influencing sex. Meanwhile, the theory is just a little worse than useless, inasmuch as it tends to complicate the breeding problem and produces no practical results.

Another of the superstitions is that a female well nourished during pregnancy tends to produce a preponderance of females; an ill-nourished female, to produce a preponderance of males. If the theory had any validity at all, the results would be reversed because the male fetus is less viable than the female and would be more likely to succumb to malnutrition. If such a method were efficient at all, it would exercise a selective tendency by destroying the fetus of the weaker sex, and the resultant preponderance of the stronger sex would be due to fewer (and only the stronger) fetus reaching their full development. The same result could be more sanely attained by destroying at birth puppies of the unwanted sex.

The sex of the puppy is determined at its conception and cannot be altered thereafter. When the ovum and sperm, each with its haploid set of chromosomes, unite to form the zygote, with its diploid set of chromosomes, the die of sex is cast. Just how and why that is true, we shall set forth here as simply as may be.

Each diploid set of chromosomes contains one pair which are known as the sex chromosomes. As we shall see, this pair is homologous in the female and non-homologous in the male. In the cells of the bitch before reduction division, the two sex chromosomes are alike, the X-chromosomes. In the cells of the male dog before reduction division, the sex chromosomes are different one from the other, one of the two being an X-chromosome and the other being a Y-chromosome, smaller in size and different in shape from the X-chromosome.

In the reduction of the female germ cell from the diploid to the haploid chromosome count, it will be remembered that one of each pair of chromosomes is retained in the ovum. Therefore, each ovum has one X sex chromosome in its haploid set.

In the first reduction division of the male germ cell (the primary spermatocyte) to form the secondary spermatocytes, which again in turn divide by mitosis to form spermatids, the X sex chromosome goes into the haploid set of one secondary spermatocyte and the Y sex chromosome goes into the haploid set of the other spermatocyte. Thus of the billions of spermatozoa which are produced in the testes of a dog in his lifetime, exactly half of them contain one X sex chromosome to each spermatozoon, and the other half contains one Y sex chromosome to each spermatozoon.

Since the ova all contain one X, and since 50% of the sperm each contain one X, and the other 50% of the sperm each contain one Y, when an ovum is fertilized by a sperm to form the zygote it must contain either two X's or one X and one Y. If it contains one sex chromosome of each kind, an X and a Y, the puppy will be male. If the diploid set of chromosomes in the zygote contains two X sex chromosomes, the resultant puppy will be female.

(The phenomenon of sex determination is here described as it pertains to the dog, although the process is the same in most mammals, including man, and in many other animals, among them our friend Drosophila. In many birds and moths, the process is reversed, the sperm all having like chromosomes and the ovum at its reduction keeping one of the alternates of the pair. In some species, the Y chromosome does not exist at all, one haploid set after reduction in the male containing one X chromosome, the other set containing no sex chromosome at all. The results, however, are approximately the same.)

Since the male produces X and Y bearing sperm in equal numbers, the theoretical probability that the ovum will be fertilized by an X-bearing sperm is equal to the probability that it will be fertilized by a Y-bearing sperm.

In man, for whom we have more accurate statistical data than for the other animals, it appears that more ova are fertilized by Y-bearing (male) sperm than by X-bearing (female) sperm. This, despite that the numbers of sperm produced of each kind are exactly equal. It is reasoned that the Y-bearers are perhaps more agile, with less density, and therefore find their way into the oviducts in greater numbers, or that the ova are less resistant to their penetration. At any rate, a few more boys than girls are born.

However, it is also known that more male fetuses die and are aborted than female, that more males die at birth, and that more die in the early part of life. A somewhat greater percentage of males are born alive than females; but, in man, the proportion of sexes is reversed by the larger survival of the females. In fact in man, the number of males dying in the first seventy-two hours of life is twice that of the females for the same period.

All of these facts appear to hold good for most mammals and, except the last fact, for the dog. However, in the dog under domestication, except in the best purebred strains where bitches are considered for their breeding value, the preference for males is so great that better care is given them as a lot and more males reach maturity than females. A census of the domestic canine population would doubtless show a great preponderance of adult males over adult females. This is due, if not to direct female infanticide, to neglect of the young of the troublesome sex.

From the time of conception until late in life the male is less resistant to unfavorable elements in the environment, is, indeed, the weaker sex.

The X-Y combination, while somewhat more frequent, is not so conducive to the stamina of the zygote as is the combination of X-X. The result is that, while slightly more males are born than females, yet the greater resistance of females to unfavorable environment tends to balance the sexes numerically. In the domestic dog, preference for males as pets prompts owners to offer better protection and care to them, and weighs the numerical balance.

For practical purposes, the breeder may well assume that, while his bitches may produce some litters preponderantly or wholly of one sex, in the long run, when the average of a great many litters is taken, it will be found that the sexes are of approximately equal numbers. There is, furthermore, as yet no practical method known by which that ratio may be altered.

The sperm which carries the Y chromosome is smaller than the sperm which carries the X chromosome. This smaller size is sometimes put forward as a possible reason for the greater agility or better ability to penetrate the covering of the ovum, or both, which would result in a greater number of males being conceived. However, it does not seem possible that such is the case, for if the sperm had any such positive advantage we would be forced then to explain how any X-bearing sperm preceded any Y-bear-ing in the fertilization of the ova. Without wishing to labor the point, there is no question that the Y-bearing sperm have a slight advantage. However, such a small advantage is more likely to be due to the receptivity of the ovum membrane with respect to X and Y sperm than to any size, agility, or density of the sperm.

The recognition of the difference, minute as it is, in the size and density of the two chromosomes and of the sperm which carry them, has led to a method of separating the Y-bearing sperm from the X-bearing sperm by centrifuging the semen. Unfortunately, the sperm, though separated, are killed in the process. If after such a separation live sperm were available the breeder could use only the sperm to produce whichever sex he might choose in any mating and thus it would be possible for him to produce males or females at will. Such a method would, of course, require artificial insemination, which we already know to be practically possible.

Genetic workers for the Soviet government have brought about the segregation of the two kinds of sperm by use of an electric field but, again, at the cost of killing all of the sperm.

A process to influence sex at conception would be a boon to breeders of all kinds of domestic livestock and would assume great economic importance. It is, of course, to be feared that short-sighted greed might prompt breeders to the production of animals of only the more marketable sex, which would result in a dearth of good breeding animals of the alternate sex. Unless wisely used, it might bring about deterioration rather than improvement of the breeds.

While we are not as yet able to alter the ratio of the sexes, we may calculate that over a long period a group of dogs will turn out approximately the same number of puppies of each sex, with a slight tendency toward a preponderance of males.

We often see a statement that a given male dog sires numerically large litters in which males greatly preponderate. The implication is that the male at least in part determines the number of puppies in the litter and how the litter shall be divided as to sex. We know, and know why, such claims for a dog are specious. We know that the size of the litter is entirely dependent upon the number of ova deposited in the oviducts which are fertilized and develop. (A single ovum may rarely produce identical twins.) And we know that if a male dog deposits enough live and viable sperm to fertilize one ovum, he deposits enough to fertilize a thousand or a million ova. We also know that while the sex of the zygote hinges upon which kind of sperm, X-bearing or Y-bearing, fertilized the egg, the numbers of each kind of sperm are the same, and which kind shall fertilize any given ovum, or the proportions of which kind shall fertilize any set of ova, is a purely fortuitous matter. Therefore, provided that it is producing live and vigorous sperm at all, neither the number of puppies in the litter nor the proportion of the sexes is determined by the sire.

We have said that the sex of the zygote is determined at time of conception by which kind of sex chromosome is carried by the fertilizing spermatozoon. While that is true, it is necessary to say that in the very earliest stages of the embryo, sex does not yet manifest itself. It is, so to speak, on the fence and the sex chromosome in the sperm seems to topple it toward maleness or femaleness.

The early embryo contains the potentialities for either sex, and, indeed, the developed organism, of whichever sex it may be, harbors within itself in vestigial and miniature form the sexual organs of the opposite sex. The early embryo is hermaphroditic and the sex chromosomes determine which sex shall develop.

Thus we see that sex is essentially a Mendelian factor, determined by the sex chromosomes and the genes within them. The female is homozygous for sex, with two X-chromosomes in the zygote. The male is heterozygous for sex, with an X and a Y chromosome in the zygote.

Gynanders, organisms in which one part of the body is of a different sex from the rest of the body, occur among insects but are not known in dogs. Hermaphrodites and intersexes occur in dogs so rarely that no cognizance need be taken of such conditions by the practical breeder. In them the sex chromosomes have gone, somehow, astray. It should be needless to say that if such monstrosities are permitted to survive, they should not be used for breeding, even in such cases as it might be possible to do so.

There are no recognized, specifically sex-limited characteristics in the dog, such as antlers, beards, crests, and manes, which appertain to one sex alone and are known as the secondary sex characters. Of course, the primary sex characters, the genitalia themselves and the developed mammaries of the female, are sex-limited, but they are a part of sex itself.

The so-called sex-limited traits, secondary sex traits, are governed largely or entirely by the hormones secreted by the gonads, which is indicated by the inhibition of their development by early castration.

While there are no specifically sex-limited, secondary sex traits in the dog, there is the matter of sex character, femininity in the female and masculinity in the male, which the breeder must consider. Sex character (to be distinguished from secondary sex traits) is presumed to depend not entirely upon the hormones from the interstitial glands of the gonads, but also upon the hormones secreted by some of the other endocrine glands, particularly the pituitary and the adrenal cortex.

The size and functioning of such glands is a heritable matter and the sex character of breeding stock is, therefore, to be considered.

In no variety of dogs is the disparity of type between the sexes so great as to unfit either sex for the work and service for which the variety was intended or is used. In a few breeds, notably Bulldogs, French Bulldogs, and Chow Chows, the more closely the bitch approaches to the ideal of the male dog in type, outlook, and character, the better she is considered to be. In other breeds, the difference in sex character is small, a degree of daintiness, and added refinement of contour making for femininity in the bitch; an added aggressive outlook, a stallion-like carriage of neck, and a slightly greater size and substance mark the masculinity of the male. This difference is a subtle one and should result in neither weakness of the bitch, on the one hand, nor in coarseness, grossness, or commonness of the male on the other hand.

The slightly doggy bitch is usually and rightly preferred over the bitchy dog. Good judges like neither, and most expert breeders choose to use for reproductive purposes animals which approach in sex character as nearly as possible to the ideals of their sex and breed.

Sex-linked traits are not to be confused with sex-limited traits. (See the glossary if there is any doubt.)

The sex chromosomes carry more genes than merely the ones which determine sex, and it is the other genes in those chromosomes which account for the traits known as sex-linked. A sex-linked trait occurs much more frequently in one sex than in the other, but may occur in either sex. Examples are deafness in albino cats, red-green color blindness, and hemophilia in man, and some color manifestations in chickens.

There are no such sex-linked traits recognized in the dog, although there is little doubt that they exist. This interesting phenomenon of sex-linked inheritance need not concern the breeder of dogs unless he be crossing breeds for the purpose of establishing a new breed. In that case, some traits which are so deeply established in both sexes of our established breeds as not to be recognized as sex-linked may reveal themselves in their true colors.

We are already so well supplied with canine varieties that the hybridization of the established breeds for the purpose of making another breed is not to be encouraged. Therefore, there is little to be gained from any discussion of sex-linkage here, except to warn against confusion of it with sex-limitation. The breeder who desires more information upon the subject can easily find it in any modern text book of genetics. While somewhat intricate in its explanation, the phenomenon is not abstruse or difficult of comprehension.

It is by means of sex that the dog reproduces its species. It is, therefore, the instrument which the breeder of dogs must manipulate to achieve his end. A knowledge of sex and its manifestations will do much to clarify his procedure. However, discussions of details which can have no practical application to the breeding of dogs are omitted here as more likely to confuse than to clarify.

The behavior of the sex chromosomes, as distinct from the other chromosomes, the autosomes, shows us that we are not able as yet to influence the sex of our puppies, and the reason why we are not. The breeder of good dogs has more to concern him than the sex of his puppies. Dogs or bitches, they are equally welcome—if only they are good enough—whichever they may be.

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