Chapter 6 - NeoMendelism

When Gregor Mendel in 1866 published his laws that revolutionized the science of genetics, he was unaware that all the physical attributes an organism is heir to are transmissible from generation to generation in consonance with those laws. The great mass of evidence accumulated since the rediscovery of Mendelism in 1900 all points to the conclusion that every trait of every part of every organism is the result of the presence of the gene for some Mendelian factor or its allelomorph. The single possible exception so far observed is in the color of certain plants, the leaves of which are green, white, or mottled. This condition seems to be produced by certain so-called "plas-tids" in the cytoplasm of the cell and to depend only in part, if at all, on the chromosomes. This single exception need not confuse the breeder of dogs or turn him from the full acceptance of Mendelism. The testimony of the whole faculty of authoritative geneticists is for the universality of the Mendelian laws.

Mendel's garden peas fortunately had characters, manifestations of which were dependent upon single pairs of allelic genes, and the genes for these various sets of characters were in different chromosomes of the haploid set. Else he might never have reached his conclusions. We have seen in the previous chapter that only "crossing-over" saves from invalidation the Second Law as it pertains to such genes as may be found in any single pair of chromosomes.

The discovery of the chromosomes and genes and their behavior was made long after the death of Mendel, who was unaware of the material entities which carried his factors from one generation to another. His experiments with hawkweed and other plants were in their results baffling to the point of discouragement. What he thought were individual factors refused to split out in the same ratios as in peas.

We now know that most traits which appear to our observation as single attributes in the phenotype of an organism depend genetically upon two or more sets of genes, which sets may be in separate pairs of chromosomes. What seems a single factor is in reality a complex of factors, and the phenotypic attribute may appear only with the dominance of both or all, the recessive ness of both or all, or the dominance of one or some and the recessive ness of one or some of the members of that complex. We now know that Mendel was dealing with two or more sets of alleles in hawkweed when he believed he was dealing with a single set.

Then the presence or absence of what seems to us a single unit attribute is in reality the result of dihybrid or polyhybrid factors.

This complicates the Mendelian situation seriously. In fact, Mendel himself could not figure it out. How easy it would be for the dog breeder if there were a single set of genes which determined whether the head in his variety should be good or bad, another set which declared for or against correct fronts, another pair for correct and incorrect coat, and so through the organism. Might we not go further and wish for a single set of genes which would determine whether the whole dog should be good or bad? Of course, such a conception is preposterous.

We might wish that we could breed together Pekingese and Great Danes and be able in the second filial generation (F2) to sort out the respective varieties in as pure a form as if the mating had never been made. To say that it is impossible is not categorically true, for once in a number of experiments, to be expressed only in some weird astronomical figure, such a phenomenon might occur. But in the next million years no dog breeder should expect to see it.

It is worthy of repetition that the breeder of dogs is dealing not with whole single organisms but with thousands of sets of reciprocal, allelic genes. Which members of any of those thousands of sets is in a dog's chromosomes may determine what he shall look like and what he may transmit to his progeny.

Whether a dog's eye shall have pigment or not, whether he shall have yellow or brown eyes, or china or wall eyes, may be due to the presence or absence of a single gene for each eye. But how deep that pigmentation may be probably depends upon which of several genes, which make for intensification of pigmentation, may be present or absent. Thus, the right member of each of several pairs of genes is probably required to produce the darkest eye color.

In fact many traits are the result of gene complexes, i.e., two or more allelic pairs. Eye color appears to be the result of one major allelic pair, the "determiners," and at least three minor allelic pairs, the "modifiers."

Indeed, eye pigmentation in the dog may behave as does skin color in crosses between Negroes and whites. In that cross, there are known to be at least two sets of factors involved, the dominant genes of either of which produces dark color. When both genes for color are doubly dominant in the zygote, we have the darkest color of the pure Negro. Mulatto parents (by mulatto is meant the first cross between purebred Negro and purebred white) may then produce progeny of any of nine intensities of skin color, ranging from the darkest color of the pure Negro to the lightness of the white parents of the mulat-toes. Such extreme extractives are genetically pure and will breed as true for pigmentation of skin as if there had been no introduction of the genes from the opposite race in the mating . That does not, of course, imply that such a person, purebred for skin pigmentation, would not have other attributes of the opposite race. That would be as weird as to expect to sort out in the F2 generation purebred Pekingese from the Pekingese-Great Dane cross.

In Drosophila alone there are at least fifty genes recognized to affect the different eye colors.

Moreover* the manifestation of an attribute in the phenotype may be due to the dominance or recessive ness of any one of several sets of genes. An instance is that there are three recognized varieties of chickens in which whiteness is due to entirely different genes.

Modifiers, which are separate genes whose results are superimposed upon specific other genes, may only intensify the effect of the primary genes (the determiners) for a given trait, or they may alter the trait completely. Such modifiers are quite inert except in the presence of the primary determiners.

There are also "supplementary factors" such as those influencing the shape of combs of fowls. To the primary genes which determine that a fowl shall have a single comb may be added either of two other genes, one of which determines that the comb shall be a rose comb, the other that it shall be a pea comb. When both the supplementary factors are present along with the factor for single comb, the fowl develops a walnut comb. Until the purity of the dominance of all of the genes for both supplementary factors is fixed through selective breeding, fowls with walnut combs are prone to produce in their progeny any of the four kinds of comb we have mentioned.

Another Mendelian surprise was the discovery of two true breeding white varieties of sweet pea which when crossed resulted in colored progeny. This phenomenon is due to the fact that color is dependent upon the dominance of two genes. If either is absent, white flowers result, and each of the two true breeding white varieties has within its chromosomes one of those dominants, but only one. When they are crossed, each of them contributes a dominant gene and color results.

An analogous, almost a parallel, phenomenon to the appearance of colored sweet peas from the hybridization of the two white varieties is the production of rabbits with agouti coloration from the crossing of the white Beveran rabbit with the white Polish (albino) rabbit. Each of these two varieties of white rabbits carries in the homozygous dominant form genes, both of which are necessary for the expression of agoutism, but the set of genes dominant in the white Beveran is recessive in the Polish, and vice-versa. When the two varieties are crossed, the resulting zygote is hybrid-dominant for both genes and agoutism is expressed in the phenotype.

It is as if self-colored Pekingese should result from the cross of an albino Peke with a dark-eyed, white Pekingese. The results of such an experiment should be enlightening.

Indeed, the elaborate analysis of the Mendelian factors which determine the expression of colors, markings, and coats of rabbits is amazing in its revelations. At least twenty-six pairs of genes were found which in various combinations determine the length, color, and markings of coat. (Both its complexity and length preclude its publication here, but an excellent presentation of the results is to be found in The Mechanism of Creative Evolution by C. C. Hurst.)

Similar extensive experiments have been made to determine the exact behavior of the Mendelian factors as they affect the expression of various lengths, textures, shape, color, and markings of the coat and skin color of the dog. This will be discussed in Chapter VII.

There are also "lethal" factors, such as the one which prevents the embryonic development of a mouse pure dominant for yellow coat color. Yellow mice never breed true for that color, which cannot be fixed. Two yellow parents, no matter how many generations of yellow in their ancestry, produce gray progeny in the approximate ratio of one gray to two yellows. The solution of the problem is that the double dose of yellow (one from each gamete), necessary for homozygous dominance of the factor, causes the death of the resultant zygote. The zygote tolerates one gene for yellow color, which produces hybrid dominance, but homozygous dominance, i.e., two such genes, will result in its death.

Whether there are lethals in the dog is not yet recognized. Morgan and his associates have found some forty lethals in Drosophila. It certainly seems reasonable to assume that in the more complex organism of the dog there are probably many lethals or semi-lethals. If such lethals exist in the dog, they need not be discouraging since they do not preclude our producing what we want except in its pure, homozygous, breeding form. It is even more likely that the lethals do not affect those traits for which selection is made in the officially recognized breeds of dogs. Most lethal or semi-lethal traits are deleterious to the organism.

The influence of the sex hormones from the gonads of the male animal upon some Mendelian traits is most interesting, as, for example, the matter of horned and hornless sheep. Both sexes of Dorset sheep have horns; of Suffolk sheep, both sexes are hornless. In the progeny of the first cross of the Dorset with Suffolk sheep, the males only are horned, the females hornless. In the F2 generation, which results from breeding a horned, crossbred ram to a hornless, crossbred ewe, 75% of the males and 25% of the females are horned, the other 25% of the males and 75% of the females are hornless.

By way of explanation, it requires both genes of the pair in the zygote to produce a horned female, whereas a single gene of the pair, reinforced by the male hormone, will produce horns in the hybrid dominant male. The horned females and hornless males are respectively recessives and pure dominants. Of the horned males in the F2 generation, 331/3% (i.e., 25% of the entire generation, horned and hornless) are pure dominants, 662/3% are hybrid dominants. So of the hornless females in the F2 generation, one-third are pure recessives and two-thirds are hybrid dominant. Thus they segregate into the expected Mendelian ratio for a single pair of factors.

In the various breeds of purebred dogs, except for somewhat greater average size in the males, and for a general masculinity of character, expression, and type in the male as opposed to femininity in the female, there are no recognized secondary sex characters. And so, such Mendelian manifestations as horns and absence of horns in sheep, baldness and haired scalp in man, beards and beardlessness, need not be sought and do not require to be avoided. The standards of the recognized breeds of dogs are the same for male and female, except that in a few of them allowance is made for difference in that well-nigh indefinable thing called sex-character.

Indeed, many of the phenomena here cited, at first consideration, have nothing to do with dogs. Since, however, the dog is an expression of a constellation of Mendelian factors, the breeding of dogs is the combination and recombination of such factors. It, therefore, is worth our while to consider the behavior of the genes in their Mendelian segregation somewhat more elaborately than merely to describe Mendelism in its simplicity as Mendel knew it.

To go into the details of what has been learned about Mendelian expression by the many workers in the science of genetics since the rediscovery of Mendel's laws would require a library of volumes of this size. These few references to the complexity of the behavior of the genes will demonstrate in part that many phenomena of inheritance, which upon superficial examination seem not to be Mendelian at all, upon careful analysis are seen to observe the Mendelian laws as categorically as Mendel's own garden peas did.

Especially must we take cognizance that a trait may appear to manifest itself as a single entity, though it may be in fact due to a complex of allelic genes. For the simplification of our task of breeding dogs, we might desire that a single gene should determine what we chose to denote as a single trait. Indeed, the great value of Mendelism in its entirety is that it brings us an understanding of the fact that most of the traits of our dogs which we look upon as units are genetically analyzable into groups of factors, all of which must, willy-nilly, be taken into account in our efforts toward the improvement of any breed of dogs.

The intelligent breeding of dogs is not all beer and skittles.

Good results are sometimes obtained by ignorant breeders through luck, persistent trial and error, main force and awkwardness; but a fickle fortune is liable to withhold her largesse from such a one at any time. The knowledge of the immutable laws is the safest guide for the breeder of animals. That the laws are somewhat complex in their application adds at once to the difficulty of breeding toward a definite ideal and to our delight when we approach that ideal. Good dogs would be good friends of men though they were easy to come by, but the difficulties and complexities of breeding them add to the joy of achieving them. Mendelism is the law which governs heredity and the breeder can violate it only at the price of his strain. Mendel no more than found the end of the yarn in the vast tangled skein of heredity. It has remained for the twentieth century geneticists to straighten the snarls, loose the knots, and bring order from the confusion of that interminable complexity. Every new yard which is untangled and added to the orderly wound ball of knowledge, every new discovery in the genetic realm, tends to confirm the now accepted belief that the whole skein of heredity is of but a single thread and that the Men-delian factors determine every aspect of every organism. In Mendelism lies the whole secret of the breeding of better dogs. How the knowledge of the Mendelian laws may serve us to that end is discussed in the ensuing chapter, "The Implications of Mendelism."

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