Chapter 4 - Genetics-What It Is

The year 1866 brought the announcement of a scientific discovery comparable in its import and implications to Copernicus' theory of the sun as the center of our universe, to Newton's law of gravity, and to Darwin's theory of biological evolution. Indeed, it is perhaps due to the concern of the world of biology with the then new Darwinian doctrine that this other, at least equally great and far reaching revelation, was so neglected as to be forgotten. That revelation is known today as the science of genetics embodying the laws of Mendelian inheritance. These laws provide the fundamental truths behind all we know about the manner in which animal and plant parents, by means of heredity, transmit to their progeny their own traits and characters and those of their ancestors.

Gregor Johann Mendel was abbot of a monastery at Briinn, in what is now Czechoslovakia. For eight patient, long years he had experimented in the crossing of distinct varieties of garden peas. Publishing the results of his research in an obscure botanical journal, he died disappointed that the world of science had taken no cognizance of what he had so laboriously learned. As Columbus did not know that he had found a new half of the world, it is to be doubted that Mendel was aware of the tremendous significance of the phenomena which he revealed. If he had known how much they meant to the practical welfare of mankind, he would surely have forced his truths upon the reluctant world with as much energy as he had used to discover them. But Mendel was a plodding scientist, not a propagandist. Perhaps he did know; perhaps it was his little joke.

Like seeds of some giant sequoia, the Mendelian laws lay dormant from the time of their publication until 1900 when two Germans, Correns and Tschermak, and a Dutchman, the great Hugo de Vries, working separately and independently of each other, rediscovered them. Researchers turned up Mendel's original publication in the Proceedings of the Natural History Society of Briinn, which of course gave to the abbot scientific priority and to the world Mendelism.

To some readers it may seem a far cry from the breeding of peas in a monastery garden in Briinn to the breeding of purebred dogs in America. The bearing of the discoveries made in those plants upon the breeding of animals is, however, direct, and the discoveries Mendel made are just as applicable to the germ plasma s of our dogs as to that of the peas in our garden.

Mendel's peas may seem, too, rather dry. But seasoned with a realization of their far reaching implications, and well masticated to facilitate their digestion, they are excellent mental nourishment for anyone who wishes to breed good dogs. Now let us look briefly at the regularities discovered by Mendel.

Mendel's first great contribution was to discover that the individual was not the basic unit of inheritance, but, rather, that each individual is composed of a number of characters which may be inherited separately. These "characters" are known as "unit characters." Mendel based his experiments on unit characters which, unbeknown to him, were to provide the key to the riddle of heredity which had puzzled mankind for centuries.

Unit characters are traits which normally appear as unmodified entities or do not appear at all and, within the given variety, they are found in contrasting pairs. The garden peas with which Mendel experimented provided contrasting pairs of unit characters: they were either tall or short and their seeds were either smooth or wrinkled. From his researches, Mendel deduced two basic kinds of inheritance. His method was so simple that anyone through controlled mating’s of plants or animals can repeat his experiments and reach the same conclusions.

First, let us take as our unit character blossom color and we will cross a pure purple-flowered plant with a pure white-flowered plant. (The word "pure" refers to the fact that both the purple and the white plants had been separately bred for some six or seven generations and had produced respectively only purple and white plants.) Mendel dutifully noted that the progeny of this purple-white cross were ALL PURPLE, i.e., all of the plants of the First Filial (the F1) generation resembled only the purple parent. Now let us take the next step and breed these F1 plants to each other (the pea is particularly good for this demonstration because it is self-fertilizing). In the Second Filial (F2) generation we do not get all purple or all white but, rather, 3 purples to 1 white, a ratio of three to one. In fact, the F1 purples will continue to produce the 3-1 ratio as long as they are self-pollinated, i.e., bred to themselves. You and I might have been content to call off the experiment at this point, but not Mendel. The abbot was an excellent scientist and he knew well that all probability factors had to be tested. He therefore proceeded to the flowers of the F2 generation.

Now let us look at the results. We note that in the F2 generation there are three purple flowers and one white. We assume that we have only two kinds of flowers, purple and white. However, what we really have with respect to heredity is three kinds of flowers. (1) The white flowers when bred to themselves produce ONLY white flowers; (2) of the three purple flowers, one of them when bred to itself will produce ONLY purple flowers; (3) the two remaining purple flowers will produce purple and white flowers in the 3-1 ratio. Genetically, the flowers of the F2 are 1/4 pure purple, 1/2 hybrid purple, and 1/4 pure white. ("Hybrids" are forms resulting from the mating of parents showing contrasted unit characters. In our illustration all of the F1 purples are hybrids.) Because in the F1 hybrids no white flowers appeared, Mendel called white recessive in contrast to the dominant purple color.

Mendel theorized that traits like flower color are passed on as units in the sex cells. One unit can be purple or it can be white but never a mixture of the two. From pure purples only purple units are transmitted; from pure whites only white units are passed on; but from the hybrids, purple and white units in equal numbers are transmitted.

The reader may clarify for himself the concepts involved here by performing the following experiment: take 100 white marbles and 100 black marbles; put them into a bag and mix them thoroughly. The resultant mixture may be considered as analogous to the F2 generation, i.e., the mixture of the germ plasma of the purebred parents with respect to the single trait of color.

Blindfold a child and ask him to withdraw marbles from the mixed lot, two at a time. While two hundred marbles is too small a number to give exact mathematical probability, the reader can see that the probability factor is that the child will select 25 pairs of marbles of which both are white, 50 pairs of which one marble is black and one is white, and 25 pairs of which both marbles are black.

If we consider the white marbles as the recessives and the colored marbles as the dominants, we will see that in the F2 generation of marbles we have the Mendelian ratio of 25% pure recessive white pairs, 50% hybrid black and white pairs, and 25% pure dominant black pairs. By now the reader should be able to calculate what results would obtain from crossing a pure white with a hybrid purple and from crossing a hybrid purple with a pure purple. (If you can't figure it out, the answer appears later in this chapter.)

We said previously that there were two basic kinds of inheritance. Let us proceed to the second type. We will state it and then give Mendel's First Law of Inheritance.

Let us begin this experiment by breeding pure white Four O'Clocks (Mirabilis jalapa) with pure red Four O'Clocks. Purity has been established by breeding whites to whites and reds to reds for enough generations to make sure that no other type appears. Instead of getting all whites or all reds in the F1, as we might have expected, we get ALL PINKS. The genetic transmission has produced in the F1 a blend of the original parental colors. Before jumping to any hasty and ill-conceived conclusions, let us breed these F1 pinks to themselves. In the F2 we get 1/4 red, 1/2 pink, and 1/4 white. Note carefully that both of the original parent colors reappeared in the F2. Several puzzling factors now become apparent: (1) the F2 pinks were hybrids carrying in their germ plasma as discrete units the red and white parental traits; (2) the "blended" effect of the F1 color did NOT involve a blending of the unit traits in the germ plasma for if the units had blended in the germ plasma the F2 would have yielded only PINKS, no REDS or WHITES would have appeared. It therefore becomes obvious that the color PINK is only in the blossom and that there is no blending of the unit characters themselves.

Perhaps one more step will bring in proper focus the form of genetic transmission. If the REDS of the F2 are bred to each other they will produce only REDS. If the WHITES of the F2 are self-pollinated, they will produce only WHITES. If the PINKS of the F2 are "selfed," they will produce RED, PINK, and WHITE in the 1/2 RED,1/4 PINK, 1/2 WHITE ratio. If the whites and reds of the F2 are bred together, they will produce pinks and you can start the whole thing all over again.

It is most important for the reader to understand that the unit characters remain discrete entities and that the blended effect is only in the blossom.

We must also note that the genetic ratios for the F2 in both of our experiments are fundamentally the same. The purple and white flowers in the F2 had a blossom color ratio of 3-1, but we saw that their genetic ratio was 1/4 pure purple, 1/2 hybrid purple, and 1/4 pure white. In the second experiment the pinks of the F1 when self-pollinated, produced the same genetic ratio: 1/4 pure red, 1/2 hybrid pink, and 1/4 pure white. What appears to be a second type of inheritance is actually a conformation to the same fundamental genetic laws.

Likewise, the roan color in shorthorn cattle is the hybrid dominant manifestation of the cross of the pure dominant red with the recessive white. The blue Andalusia chicken is the hybrid dominant manifestation of the cross of the white chicken and the black chicken of that breed.

And there are innumerable other instances that could be cited of opposing unit characters in both animals and plants: simplex-blue eyes and brown human eyes, albinism and pigmentation in all species, red eyes and white eyes in the vinegar fly (Drosophila), the normal mouse and the waltzing mouse. Such pairs of opposing factors are known as alleles or allelomorphs.

Organisms which are alike in their body characteristics, without respect to their germinal character, are said to be of the same phenotype. Organisms of the same genetic makeup, without respect to their body characteristics, are of the same genotype. Thus, all of our pure recessive white flowers have both the same phenotype and the same genotype. All of the purple flowers are of the same phenotype but the hybrid purples have a different genotype from the pure purples.

The First Law of Mendel, known as the "Law of Segregation," is as follows:

That some attributes of living organisms are due to factors (dominant and/or recessive) in the gametes from which the organisms developed; that these factors do not blend but are individual and indivisible entities; that two parents with any factor as pure dominants in the gametes from both will produce in their immediate progeny pure dominants only, as pertains to that factor; that two parents with any factor as recessive in the gametes from both will produce in their immediate progeny only recessives as pertains to that factor; that in the crossing of parents, one pure dominant as pertains to the given factor, the other recessive for that factor, all of the immediate progeny will be hybrid-dominant for that factor; that two hybrid-dominant parents will produce progeny in the approximate ratio (large numbers of progeny to be considered): 25% recessive, 50% hybrid-dominant, and 25% pure dominant.

In the statement of this law, the phrase "some attributes of living organisms" is deliberate because Mendel, when he promulgated the law, was unaware that it might apply to all attributes of all living matter. Indeed, it is impossible to make such a categorical assertion even at this time, but it is the opinion of most workers in genetics that, with the possible unexplained transmission of leaf color in a few plants and a very few other similar phenomena, every attribute that flesh is heir to is, in all likelihood, the result of Mendelian inheritance. Whether a cat is to be black or white, whether a horse is to be fast or slow, whether a man is to be musical or unmusical, short or tall, blond or brunette, whether a dog shall have a long tail or a short tail, whether a dog shall bay on the trail or keep silent; all, everything that any living organism is or can become, is believed, by those who best should know, to be inherited in the Mendelian manner.

Mendel made further experiments with peas, considering at least seven sets of allelic factors. He found tallness of plant dominant over recessive dwarf ness; unripe, green seed pods dominant over yellow seed pods; inflation of pods between seeds dominant over recessive constriction of pods; flowers borne along stem axis dominant over recessive flowers bunched at the top of the plant; colored seed skins dominant over recessive, white seed skins; yellow cotyledons dominant over recessive, white cotyledons; and smooth seed dominant over recessive, wrinkled seed.

It is fortunate that Mendel's experiments were made with a plant in which he was able to distinguish so many simple alleles dependent upon a single factor for their phenotypic traits. Had he chosen to work with organisms more complex in the genetic requirements for each phenotype, he might never have arrived at his conclusions. Indeed, it is possible, even likely, that other workers before Mendel, who had experimented with the breeding of other organisms with equal zeal and equal acuity of observation, might have discovered his laws but that the genetic transmission of their experimental material was so involved as to obscure the facts. This we shall see in the discussion of "Neo-Mendelism" in Chapter VI.

Now, obviously, there is more than one pair of unit characters going into the hereditary makeup of plants. Mendel knew this and he knew, too, that eventually he would have to solve the problem of following two unit characters at a time, of how they are transmitted, and of how they behave relative to each other. By solving this problem, Mendel arrived at his Second Law, the Law of Independent Assortment.

We will illustrate the law with one of Mendel's own experiments. In working with his peas Mendel crossed a plant that was pure strain for "tallness" and for "yellow seed" with a plant that was pure strain for "dwarf ness" and for "green seed." Two unit characters were involved here, the allelic structure being: tall with dwarf, and yellow seed with green seed. Every plant in the F1 generation was tall and had yellow seeds. Now look at the diagram on page 62 and note that every plant in the F1 is a double hybrid, i.e., it contains in its germ plasma the unit characters TALL, Dwarf/YELLOW SEED, Green Seed, with TALL-YELLOW SEED manifesting as dominant in the pheno-type. (Recessive traits are shown in lower case letters.) Remember, also, that only two traits are involved, size (TALL-dwarf) and color (YELLOW SEED-green seed) and that each trait has a single contrasting pair of unit characters.

Let us proceed to self-fertilize these double hybrids of the F1 generation and observe the results. The F2 generation yielded four distinct plants in phenotype: (1) TALL/YELLOW SEED; (2) TALL/green seed; (3) dwarf/YELLOW SEED; (4) dwarf/green seed. These four phenotypes were in the ratio of 9:3:3:1, respectively.

Now bear with us and we'll do the whole thing once more, slowly, and this time with pictures.

                                                                  Unit Characters
1) The Parental Cross:                                TALL/YELLOW SEED x dwarf/
                                                                  green seed
2) The F1 Generation:                                 (all plants) TALL/YELLOW

SEED and dwarf/green seed
3) The F1Generation self-fertilized:              TALL: dwarf/YELLOW SEED:
                                                                  green seed x TALL: dwarf/
                                                                  YELLOW SEED: green seed
 
4) In the F2 all possible combinations appear by pure chance. The possible combinations being:
 
a) TALL/YELLOW SEED                   9
b) TALL/green seed                             3
c) dwarf/YELLOW SEED Ratio           3
d) dwarf/green seed                              1

Note: This is an invariant ratio when pure strain double hybrids are bred together.
 
The following diagrammatic table will show how the genotypes of the 16 plants are distributed, each square representing an individual plant.

Dominants T-Tall Y-Yellow Seed
 
Recessives D-Dwarf G-Green Seed

Thus we can see that each plant has received one factor (unit character) for each set of alleles from each parent. (It is important to recognize that allelic unit factors cannot be passed on together. For example, TALL and dwarf cannot be transmitted as a single combination, nor can YELLOW SEED and green seed.) The presence of a single dominant unit factor determines the manifestation of that dominant trait of the allelic unit in the phenotype. When dealing with pure strain dominants and recessives, as we are here, recessive unit factors must be in "double doses," i.e., one derived from each parent, to manifest in the phenotype. Where the unit factors in the genotype are a contrasting pair, i.e., one dominant and one recessive, the recessive trait remains buried in the genotype and the dominant trait manifests in the phenotype. That buried recessive, however, is not discarded from the genotype pattern for it stays there and in the next mating if it pairs with another recessive of the same kind that recessive trait will now appear in the phenotype.

When two sets of alleles are involved in a cross, it is called a dihybrid cross and the plants of the Fi generation are called dihybrids. When three allelic sets (three unit characters) are involved, the plants of the Fi generation are trihybrids. When more than three sets of alleles are involved, the resultant generations are polyhybrids.

As we have seen, a dihybrid cross in the F2 generation produces 4 possible phenotypes in the approximate ratio of 9:3:3:1, and it produces 9 genotypes, of which only one is recessive for both factors, and one pure dominant for both factors. The remaining 7 are hybrid dominant for one factor or for both.

A trihybrid cross produces 8 phenotypes with 27 possible genotypes, the ratio of the phenotypes being 27:9:9:9:3:3:3:1. The possible genotypic patterns in the trihybrid cross could be illustrated in a diagram of 64 squares, as is the dihybrid cross in the 16 squares of the diagram on page 62.

Recapitulating, the Second Law of Mendel, or the Law of Independent Assortment, is: "... when double hybrids are bred together all possible combinations occur in the offspring by pure chance." We shall subsequently see that this Second Law of Mendel is not categorically valid. However, before we can understand why it is not absolutely valid, we need to discuss the chromosomes and their genes, which we shall do in the next chapter.

If all of these figures, squares, and ratios have seemed tedious, we excuse ourselves by saying that they are the very flesh of the skeleton of Mendelism. They must be understood before we can turn our attention to matters of more far-reaching and practical consequence.

The importance of "independent assortment" is set forth by Dunn and Dobzhansky in Heredity, Race, and Society:

"This independence in inheritance of separate traits of plants, animals and men was an entirely new idea which Mendel was the first to recognize and to prove. It has many important and interesting consequences. For example when Mendel crossed two varieties of peas, one with yellow round and one with green wrinkled seeds, he obtained four varieties in the second generation. The two new ones had yellow wrinkled and green round seeds respectively. Thus new varieties can be obtained by crossing, and the number of kinds from which the farmer or gardener may choose can be greatly increased. This is of great importance in practical agriculture. Suppose that you have two varieties of a crop or vegetable plant, and that each variety possesses an advantageous trait which the other does not have. For example, one variety may yield well but may not have sufficient resistance to frost, while the other is frost resistant but produces unsatisfactory yields. It is obviously desirable to have a combination of good yield and frost resistance in the same plants. If yield and resistance are chiefly influenced by two pairs of independent genes, then according to Mendel's second law, this may often be accomplished by crossing the two varieties, and selecting in the second generation of hybrids the plants which possess the desired combination of traits. Plants in which the undesirable qualities of the initial varieties are combined will also appear among the hybrids, making a careful examination of individual plants and their progenies essential. Many of the best varieties of cultivated plants have been arrived at by combining through appropriate crosses the desirable traits which were present separately in several older varieties."

Even though Dunn and Dobzhansky have used plants to illustrate the importance of independent assortment, their paragraph is equally applicable to animals.

For long ages it was assumed that the germ plasma of the parents blended to form a new generation as if one mixed water and wine, and that the child would manifest attributes somewhere intermediate between the analogous attributes of the parental pair. It seemed like common sense but, like much else that we have accepted as common sense, upon investigation it turned out not to be true. It is one of the "Things That Are Not True," discussed in Chapter XIV, of that title.

Rather than being like the mixing of wine and water, the breeding of plants and animals is like mixing together black and white beads which can again be segregated at will.

That, indeed, is Mendelism—the fact that parental germ plasma s do not blend irrevocably together like wine and water but rather that the factors lie side by side, complete entities, un-blending and indivisible in the germ plasma , like different colored beads in a box.

This is Mendelism, Mendelism as it was set forth by the Abbdt of Briinn in 1865, forgotten until 1900, rediscovered and promulgated at the beginning of this century. However, Gregor Mendel saw the truth but through the glass darkly.

Using the two Mendelian laws as a point of departure, the twentieth century workers in the science of genetics, no less patient, no less careful, no less wise, have gone far. Every year new genetic truths are revealed, startling in their implications. So much is known now that, if adequately applied, man would be so able to hasten and direct organic evolution that it could move in a decade as far as it has previously moved in a millennium, and, moreover, could be forced to move in the desired direction. Man, through selective breeding, can make of his plants, his animals, and his descendants whatever he may wish them to be.

What Mendel saw so darkly, the modern geneticist sees clearly, ever more clearly. The new knowledge which has grown from the roots of Mendel's discoveries is called Neo-Mendelism, which will be discussed in Chapter VI of that name.

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