Now that we have seen how life is handed on, we can go on to discover how all those things that distinguish horses, men, and marigolds from each other are passed on from generation to generation. To do this, we must switch on the high power of the microscope and take a look into the very private life of the cell. With this great magnification, the cell becomes quite different—just as the ground looks different when you are flying at 500 feet from what it does when you are flying at 5000.
Let us suppose that, under this magnification, we are looking at a fertilised egg—the single cell that, made out of the union of the father’s sperm and the mother’s ovum, is the starting-point of the new colony. A short time after the sperm and the ovum have united to form the fertilised egg, you will see, more or less in the middle of the egg, a pattern that looks rather like two old-fashioned armies forming up in battle array opposite each other. The two armies, though, are exactly like one another. The first company of each is a
straight line, the second company of each a half-moon, the third is curly, the fourth a sort of blotch—and so on.
These companies are known as chromosomes, and you may fairly look upon them as the all-important organisers of cellular activities. You can here disregard the rest of the cell (cytoplasm), and fix your attention solely on these chromosome armies, particularly remembering that they are exactly the same size, shape, etc., and that one of them is the father’s contribution to the new individual, the other the mother’s. This is important because, as you will see, it is one of the things that show that father and mother play exactly equal parts in the making of a child.
Now, if we were looking at a human egg just after the union of sperm and ovum, we should see 24 chromosomes in each of the two ‘armies ‘—48 in all; if it were the egg of a certain kind of fly (Drosophila), the numbers would be 4-8, of the green pea 7-14. The numbers, shapes and sizes vary with each species—of which, in fact, they are the cause. We, however, need not consider these specific differences, and can pretend that each army is composed of one company only— that on the right from the father’s sperm, that on the left from the mother’s ovum.
The next thing to happen is rather startling—each chromosome splits neatly in half all the way down the middle (B). After that, the halves separate completely (C) and, following this, the rest of the egg starts to pinch together (D), finally becoming two separate but attached cells (E). The last stage is exactly the same as that of our one-celled creature just before it splits completely in half; but this time the greater magnification has enabled us to see the chromosomes.
We are not now looking at a one-celled creature, however, that splits into two entirely separate individuals, but at the first stage of a many-celled creature’s life. So the two cells, each the identical twin of the other, remain stuck together; and the next step is for each of them to divide again in the same way. Thus one cell becomes two, two become four, four become eight—and so on until, when the number has reached many millions, the fully grown human being (or other creature) is perfected.
This is a compressed description of cell-division, which is the basic principle of both growth and breeding. The main characteristic (setting aside some exceptions that are momentarily unimportant) is the behaviour of the chromosomes,
since, though the rest of the cell is very often unequally divided, those dictators of development are always halved with great precision—each of the two joint managing directors of the firm splits into two whenever a new branch is to be started. This means that in every cell of your body and brain there lives and moves that fatal pattern your parents stamped in the egg from which you sprang.
HOW WE HALVE OUR HEREDITY
Now return to the beginning. The chromatin (chromosome-stuff) of a one-celled creature splits neatly in half, and the rest of the cell makes a very bad attempt at doing the same. The badness of the attempt, though, does not matter, since the chromatin, like any good board of directors, can soon collect and organise local supplies; so that before very long the two new creatures are exact duplicates of each other and of their joint parent.
The growth of a many-celled creature differs only in that the cells remain stuck together, instead of swimming apart, and that the cytoplasm (but not the chromosomes) of each cell specialises in one particular job—forming the chalky substance of a bone or of a lobster’s shell, the soft, porous substance of our lungs, the elastic quality of a muscle fibre, and so on.
Now if a one-celled, a-sexual creature splits in half in order to ‘breed,’ it must obviously give exactly half its
chromatin to each of its two ‘children ‘—splitting both ot its chromosomes. But what would happen if many-celled, sexual creatures did the same? The ovum would carry two chromosomes, so would the sperm, and if the two united, the resulting egg-cell would have four chromosomes. In the next generation there would be eight, in the one after, sixteen, and so on until the egg-cell was carrying an infinite number of chromosomes. Many-celled, sexual creatures had therefore to invent some way of cell-division different from the ‘breeding ‘of the one-celled creatures or from their own bodybuilding process.
Instead of doubling the chromosomes, as in ordinary cell formation, they divided them in making gametes (a convenient term, meaning ‘marrying cells,’ to cover both ova and sperms). Now switch the microscope on to the reproductive organs of any of the higher creatures, male or female, just as ovum or sperm is being formed. The first picture you will see is the familiar one of the chromosomes ‘setting to partners,’, but from this point the dance is different. Instead of each chromosome splitting neatly in half, the two waltz away from each other (B), then the rest of the cell pinches in between them (C) and finally, the two halves of the cell become completely divided (D). There are thus two ‘marrying cells ‘or gametes, each with half the number of chromosomes of its parent.
Though ovum formation and sperm formation differ in certain minor ways, the description just given is in essence true of both of them.
Since an egg-cell, which is the start of a new individual, is formed by the union of,sperm and ovum, four different kinds of egg-cells are possible—AC or AD, and BC or BD. That cell divides into two by the ordinary splitting process, where each chromosome is neatly halved. Those two split again into four, and so on—each new cell being BD, like the original egg-cell. Thus the new individual possesses in every cell of his body and brain one chromosome of his mother’s two, and one of his father’s two.
Now going backwards a step, the chromosome from the mother came from her father, and the chromosome from the father from his father. Therefore our new individual has inherited the chromosomes of its two grandfathers in this case, though the combination might equally well have been AC, AD, or BC—both grandmothers, maternal grandmother and paternal grandfather, or maternal grandfather and paternal grandmother.
And now going forward a step, when our new individual comes to the point of breeding, it can hand on to its child either the chromosome it received from its mother, or the one from its father; but not both. Its gametes or ‘marrying-cells ‘will each be a chance-chosen half of its two parents’ chromosomes.
This is the essential mechanism of heredity, as the microscope shows it. But the theory of heredity, a sort of prophecy of the microscope’s revelations, was discovered many years before (in 1865) by the Austrian monk Mendel, who deserves to rank with his great contemporary Darwin. Unfortunately neither knew of the other, and Mendel’s combination of careful breeding experiments with sound reasoning was unappreciated for over thirty years.
HOW MENDEL SAW HEREDITY AT WORK
MENDEL actually worked on the green pea, but since he chanced on a minor complication (only later understood) it is easier to understand his theory by taking an example
from the farmyard, meanwhile pretending that Mendel himself is at our elbow. There are two colours of Shorthorn cattle, red and white. If you mate a red with a white, all the calves will be roan, a colour resulting from an intimate mixture of red hairs and white hairs. (This, you will notice, is rather like the microscope’s picture of an individual with one of its mother’s and one of its father’s chromosomes in each cell.)
Now comes the interesting part. If two roans are mated, their calves will be : ONE RED Two ROAN ONE WHITE
The Red, if mated to another Red, will breed true; and so will the White, if mated to another White.
The two Roans mated together repeat their parents’ performance (1 Red, 2 Roan, 1 White). This seems rather confusing until you argue, as Mendel did :—
It takes two parents to make one child.
Therefore each gives half of the essential birth-factors (‘genes ‘as we now call them.
Mendel put forward this gene theory as the only reasonable interpretation he could find of the results of his breeding experiments, and he did not do so until he had tried it out very thoroughly with several different pairs of contrasting characters, getting the same result each time. Then he prophesied what to expect from crossing Roan with Red and Roan with White,1 and the subsequent experiments confirmed him. And he was again justified when he tried crossing two and three pairs of contrasting characters at once. His theory worked.
Without considering these complexities, though, we can now go farther than Mendel himself, partly because many biologists have since confirmed and extended his experiments, using all sorts of plants and animals, but mainly because the chromosomes which the microscope has since revealed behave in exactly the same simple way as his theoretical genes.
Mendel said that the two parents must each give a gene to the child. The microscope shows that they each give a
chromosome. Mendel said that the two genes do not get mixed up together, but part company when the child breeds. The microscope shows that the chromosomes do retain their individuality, and that each passes into a different gamete.
In fact, if you will compare the picture of chromosomes in this section with Mendel’s pictures of genes, you will find that they are interchangeable (though Mendel, of course, did not even touch on what happens to the genes in the business of body-building. That is purely the discovery of the microscope).
We can now put the whole thing in a nutshell :—A living thing is, in a very real sense, a dual personality, since each of its cells contains one chromosome from its father and one from its mother. Its method of breeding is to break off a living fragment of itself, a half cell containing either its mother’s chromosome or its father’s—a matter of chance—but never both. That living half-cell unites with one from the other parent, and the new individual thus formed starts on its career with the full complement of two chromosomes, one from each of its parents.
If the characters of the parents contrast, like red and white, then they will struggle for mastery in the child, as it were, and produce an apparent blend such as roaning. But they never effect a permanent blend; and they part company when the child itself breeds. A roan cannot hand on roan to any of its children, but only a red gene or a white. The roan child of roan parents does not inherit the colour from either of them; it takes red from its father and white from its mother (or vice versa), and so compounds the mixture afresh.
There are a great many simple mendelian characters known in all forms of life. In men, for instance, if a pure blue-eyed man mates with a pure dark-eyed woman, their children will all have eyes of the hazel or light-brown type. And two hazel-type people will have one blue-eyed child, two hazel, and one dark. Curly hair mated to straight hair produces wavy; and two wavy-haired parents will have one curly-haired child, two wavy-haired, and one straight-haired.
This ‘simple mendelian ratio ‘of 1:2:1, by the way, only holds when large enough numbers are bred, since the shuffling of the chromosomes is a matter of chance, which only works out evenly in the long run. If you and a friend,
for instance, tossed pennies a hundred times, you would get this result—as near as no matter : 25 both coins Heads,
25 both coins Tails,
50 one a Head and the other a Tail. This is exactly the same as the ratio of one Red : two Roan: one White. But if you only tossed four times, you might very well get quite different proportions. It is the same with parents who only have four children. So do not count on these averages to enable you to prophesy the result of any one mating. They are useful mainly in revealing the mechanism of heredity, and next in allowing accurate forecasts of a large series of matings—for instance, in most kinds of practical breeding.
THE CHROMOSOME’S ULTRA-MICROSCOPIC PASSENGERS BEFORE going any further, it is as well to get a clear idea of what genes and chromosomes really are—since they are not the same thing. A gamete (ovum or sperm) is a rather compact living jelly of complex chemical nature. The chromosomes in it are tougher, more opaque, and even more complex chemically. If the cell is killed and stained, they show up like worms of various shapes and sizes.
Genes are, strictly speaking, theoretical, since they are too small for the most powerful microscope. But we know not only that they exist and behave exactly like the visible chromosomes, but also that they are carried in the chromosomes— many genes in one chromosome. And sometimes we are even able to say, for instance, that some particular gene is carried seven-tenths of the way down the third chromosome. The chromosome is a railway carriage, and the genes are its ultra-microscopic passengers.
It is also worth noticing that the chromosomes of an egg-cell bear no more resemblance to the red or white hairs of a cow than the directors of an aircraft company look like an aeroplane. These tiny bundles of chemicals are not animals or plants in miniature. They are the directors of a firm that builds such creatures.
DOMINANCE: THE SECRET OF ‘SKIPPED GENERATIONS’ WHEN Mendel experimented with his green peas, he had the bad luck to happen on the minor (but very common and important) phenomenon called Dominance, which con-
fused him and later biologists for a long time. He mated a tall pea to a dwarf. But, instead of the first generation being intermediate in height—as one would expect—they were all tall. When these were interbred, they produced the following offspring :
One Tall that bred true
One Dwarf that bred true
Two Tails that repeated their parents’ performance.
To us, looking at things with our present knowledge, the explanation of this result is obvious, since it is in essence exactly the same as the grandchildren of the red and white cattle—
One Red (true-breeding) One White (true-breeding) Two Roan (hybrids who, like their parents, therefore do not breed true).
The same diagram of breeding performance would fit either cattle or peas equally well. The sole essential difference between them is that, in peas, the tall gene completely dominates the ‘dwarf ‘when they come together in the same bodybuilding cells, whereas in cattle the opposing characters are of equal strength.
Mendel found several other instances in peas alone of one character dominating its opposite number (yellow over green, smooth over wrinkled), and a multitude of them have since been found in almost every kind of plant and animal. In the Aberdeen-Angus cattle, for example, the normal black colour is dominant to the unfashionable—but no worse—colour red. In man the ‘Hapsburg lip ‘(which is really a slightly overgrown lower jaw) that has characterised the Spanish and Austrian royal families, is dominant to the normal mouth.
And, generally speaking, useful normal characters are more common than defects, for the very good reason that defective individuals do not usually live long enough to breed, while healthy individuals do. Indeed, the only kind of defect that has much chance of survival is a ‘recessive ‘that can be handed on by apparently normal ‘carriers.’ But for the moment, the chief practical importance of Dominance is the explanation it gives of ‘skipping a generation,’ and of the sudden appearance of a child that is not the least like its parents or any other near relative.
RED-HAIRED CHILD FROM BLACK-HAIRED PARENTS TO take an instance that must have happened innumerable times, especially in Ireland. A black-haired man marries a flaming red-haired woman, and all their children are black-haired, since pure black is fully dominant to pure red. One of these children finds a pure black-haired mate and produces a third generation of black-haired children. One of these, who has inherited the concealed red gene also marries a black-haired mate—but one who is like herself (or himself) in having a concealed red gene inherited from an equally remote ancestor.
Few people keep records of their ancestors’ hair-colour, especially not farther back than the grandparents, so these two black-haired young people are absolutely astounded by the sudden, apparently inexplicable appearance of a redheaded child.
In fact, they have only seen an example of the sort of thing that is familiar to every practical breeder and experimental geneticist. The re-appearance of an ancestral character after several generations, due to the junction of two underground streams, is a commonplace to people who keep proper pedigree records.
A recessive (as the opposite number to a dominant is called) may, of course, get lost at the very first mating. But if there are four children in each generation, the chances are decidedly in favour of it being handed on to at least one of them. This re-appearance of a recessive from the mating of two impure dominants is the most usual (though not the only) cause of the children of any species ‘throwing back ‘to some remote ancestor, or even appearing to bear no resemblance to any member of the family!
In the examples given here, dominance is complete; and that is why they (and others) misled many early biologists into thinking that dominance was an essential part of heredity. In fact, it often does not occur at all, and at other times is far from complete. Dominance may range from the completeness shown in the Tall-Dwarf pea example to the semi-dominance of dark eyes over light and the absolute lack of dominance in the Red-Roan-White cattle—with many degrees in between.
Indeed, dominance is not strictly a matter of general geneti-cal principles at all—of the way the chromosomes behave in the actual breeding process. It is an affair of physiological
detail—of the way the chromosomes struggle or co-operate with each other in the development and functioning of the body. And these details naturally vary greatly from species to species and character to character.
In peas, for instance, tallness is certainly dominant to shortness. But in human beings, shortness normally tends to be dominant to tallness. Short parents, you will notice, often have tall children; but the reverse only occurs if there has been some developmental accident, like infantile paralysis. The white of English Park cattle is dominant to other colours, though in Shorthorns white is equal to red. In man and the Aberdeen-Angus cattle, black is dominant to red, but not in horses, which have such a complication of colour genes that the only certain thing is that chestnut is recessive to every other colour. Chestnut, consequently, is the only one that invariably breeds true.1 In rabbits, the wild colour, which is really a sort of ‘dazzle ‘mixture of several colours, is dominant to everything else.