Book - Evolution and Genetics 6
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Chapter 6 The Chromosomes and Mendel’s Two Laws
The discoveries that Mendel made with peas have been found to apply everywhere throughout the plant and animal kingdoms — to flowering plants, to mosses, to insects, snails, Crustacea, fishes, amphibians, birds, and mammals (including man).
There must be something that these widely separated groups of plants and animals have in common — some simple mechanism perhaps — to give such definite and orderlv series or results. There is, in fact, a mechanism, possessed alike by animals and plants, that fulfils the requirements of Mendel's principles.
The Cellular Basis of Heredity and Development In order to appreciate the full force of the evidence, a few familiar facts, that became known before the discovery of the mechanism in question, may be briefly reviewed.
Throughout the greater part of the last century, while students of evolution and of heredity were engaged in what may be called the more general aspects of the subject, there existed another group of students who were engaged in working out the minute structure of the material basis of the living organism. They found that organs such as the brain, the heart, the liver, the lungs, the kidneys, etc., are not themselves the units of structure, but that all these organs can be reduced to a simpler unit that repeats itself a thousand-fold in every organ. We call this unit a cell.
The egg is a cell, and the spermatozoon is a cell. Fertilization is the union of two cells. Simple as the process of fertilization appears to us today, its discovery swept aside a vast amount of mystical speculation concerning the role of the male and of the female in the act of procreation.
Within the cell a new microcosm was revealed. Every cell was found to contain a spherical body called the nucleus (fig. 29). Within the nucleus is a network of fibres ; a sa^) fills the interstices of the network. The network resolves itself into a definite number of threads or rods at each division of the cell (fig. 30). These rods we call chromosomes. Each species of animals and plants possesses a characteristic number of chromosomes which have a definite size, and sometimes a specific shape, and even characteristic granules at different levels. Beyond this point our strongest microscopes fail to penetrate.
Fig. 29. Diagram of a "typical cell," showing cell-wall, cytoplasm (with solid and fluid inclusions) and centrosome witli astral rays (doubtfully present in resting stage). In the center is the nucleus with its network of chromatin, and its nuclear sap.
Fig. 30. Diagram, slightly modified from Agar, to show a typical cell division (karyogenesis). The chromosomes are represented as black threads and rods, which pass onto the spindle fibres and then move to the poles of the spindle where they subsequently become vacuolated to form the resting nuclei of the two daughter cells.
Observation has reached, for the time being, its limit.
Certain evidence relating to inheritance through the sperm led to the conclusion that the chromosomes are the bearers of the hereditary units. If so, there should be many such units carried by each chromosome; for, the niiniber of chromosomes is hmited while the nmiiber of independently inherited characters is large. In Drosophila melanogaster it has been demonstrated not only that there are exactly
Fig. 31. Diagram to show stages in fertilization of an egg by a spermatozoon. The three polar bodies lie at one pole, and the spermatozoon is represented as entering near the opi^osite side of the egg in 1 and 2. The head of the sperm swells up and moves towards the egg-nucleus, that has reformed after the polar bodies have been given off. A centrosome forms near the sperm-nucleus. It divides into two centrosomes, which begin to se])arate as a central spindle appears between them. Around each centrosome astral rays develop. The two nuclei come together in the middle of the egg to become the segmentation nucleus. A spindle develops around the nucleus.
Fig. 32. Diagram showing the segmentation of an egg into two, four, eight cells, etc. The cells become arranged over the surface of a sphere whose interior is filled with fluid. (After Selenka.)
as many groups of characters that are inherited together as there are pairs of chromosomes, but even that it is possible to locate the hereditary elements in particular chromosomes and to state the relative position there of the factors for the characters. If the validity of this evidence is accepted, the study of the cell leads to the ultimate units about which the whole process of the transmission of the hereditary factors turns.
Before considering this somewhat technical matter, certain facts, which are familiar for the most part, should be recalled, because, on these, rests the whole of the subsequent explanation.
The thousands of cells that make up the cell-state that we call an animal or plant come from the fertilized egg (fig. 31) . An hour or two after fertilization the egg divides into tw^o cells (fig. 32). Then each half divides again. Each quarter next divides. The process continues until a large number of cells is formed and, out of these, organs mold themselves.
At every division of the cell the chromosomes also divide. Half of these have come from the mother, half from the father. Every cell contains, therefore, the sum total of all the chromosomes, and if these are the bearers of the hereditary qualities, every cell in the body, whatever its function, has a common inheritance.
At an early stage in the development of the animal certain cells are set apart to form the organs of reproduction. In some animals these cells can be identified early in the cleavage (fig. 33).
The reproductive cells are at first like all the other cells in the body in that they contain a full complement of chromosomes, half paternal and half maternal in origin. They divide as do the other cells of the body for a lonff time. At each division each chromosome splits lengthwise and its halves migrate to opposite poles of the spindle.
Fig. 33. a, Section of egg of Calligrapha bigsbyana, showing "germ-cell determinants" (granules), g c d, at posterior end of egg; b, posterior end of a later stage of same, showing primordial germ-cells; c, Section of egg of Miastor, showing single primordial germ-cell at posterior end. (After Hegner.)
But there comes a time when a new process appears in the germ-cells (jigs. 34 and 3.5). It is essentially the same in the egg and in the sperm cells. The discovery of this process we owe to the laborious researches of many workers in many countries. The chromosomes come together in pairs (fig. 34^). Each maternal chromosome conjugates with a paternal chromosome of the same kind.
Fig, 34. Diagram illustrating the two maturation divisions of the germ cells in the male. In a the chromosomes appear as thin threads (leptotene stage). These conjugate in pairs, b, beginning at the two ends of each loop. The threads contract, and a spindle appears, d, near the nucleus. The conjugating chromosomes enter the spindle, d. There they separate, e, moving to opposite poles of the spindle. The cell protoplasm begins to constrict, /. The chromosomes may without entering upon a resting nuclear stage pass onto a new spindle that has developed by the division of each of the centrosomes of each daughter cell, g. Each chromosome now splits throughout its length (equational division) ; half of each goes to one or the other pole. The two daughter cells then divide, giving four cells, each of which differentiates into a spermatozoon.
Then follow two rapid divisions (fig. 34, e-i) . At one of the divisions the double chromosomes separate (fig. 34, d-f) so that each resulting cell comes to contain some maternal and some paternal chromosomes, i.e., one or the other member of each pair. At the other division each chromosome simply splits as in ordinary cell division. In the male four spermatozoa are produced (by these two divisions) from each cell of the testis (jig. 34, i) .
Fig. 35. Diagram illustrating the two maturation divisions of the egg. In a the polar spindle is present at the periphery of the egg. The three pairs of chromosomes (bivalents) are represented in black and white; the white being the paternal and the black the maternal. In b the conjugating chromosomes have separated and are moving to the poles. In c the first polar body has been given off, leaving three single chromosomes in the egg. In c these have split lengthwise and lie off the equator of a new spindle. In e the daughter chromosomes have separated and moved to opposite poles. In / the second polar body has been given off and the first polar body has divided. Three single chromosomes are left in the egg.
In the female the two divisions of the egg-cell are very unequal (jig. 35), although the chromosomes are distributed equally to all the cells. Thus at the first division one cell is very small (jig. 35, c) and is called the polar body. At the next division the polar body divides again, and at the same time the egg divides again also, producing another polar body (jig. 35, d, e, /) . The three polar bodies and the eggcell are equivalent to the four spermatozoa, but only the egg-cell undergoes further development — the polar bodies disappear. Although only one cell survives, nevertheless there will be as many kinds of mature eggs as there are kinds of sperm cells (with respect to the distribution of the chromosomes), if, as we now know to be the case, the distribution of the chromosomes in the two final divisions (maturation divisions) are the same in the eggs and in the sperm-cells. When the eggs are fertilized, each by one spermatozoon, the whole mimber of chromosomes is restored.
The mechanism of Mcndel’s Two Laws The behavior of the chromosomes at the time of maturation of the egg- and sperm-cells furnishes a mechanism for Mendelian heredity if the chromosomes are the bearers of the hereditary elements, and if they maintain their integrity both during the resting stages of the nucleus and during their period of active division. There is a great deal of evidence from direct observation in favor of this view and there is more evidence from the modern work in heredity that points in the same direction. This evidence can not be considered here, but if it is granted that these relations hold, then the behavior of the chromosomes during maturation furnishes, as stated above, an explanation of Mendel's laws.
An example will illustrate this statement. If in the four o'clock the elements for red flower color are carried in the red parent by the two members of the same pair of chromosomes and the elements for white flower color are carried in the white parent by two members of the same pair of chromosomes, the germ-cells (ripe egg- and sperm-cells) will each carry one of these chromosomes (fig. 36) . If the red plant is crossed to the white, the pink hybrid will have a red- and a white-bearing chromosome.
When in the hybrid the germ-cells ripen, these two chromosomes, being mates, will come together as a pair and then separate at one of the two maturation divisions, and half of the eggs will contain the red-bearing chromosome and half will contain the white-bearing chromosome. Similarly for the pollen grains. Chance fertilization of any egg by any sperm will give the combinations of chromosomes that Mendel's law of segregation requires. In other words the known behavior of the chromosomes is exactly the same as Mendel's postulated elements.
Mendel's second law for the inheritance for two or more characters also finds its explanation in the behavior of the chromosomes, provided the members of the pairs of chromosomes are sorted ont independently of each other (fig. 37). For example, in the cross between yellow-round and green-wrinkled peas, if one pair of chromosomes in the hybrid (Fi) carries the contrasted elements yellow and green and another pair of chromosomes of the same hybrid carries the round and wrinkled elements, then, if these chromosomes at the maturation period behave independently, there will be four kinds of germ-cells produced. These four kinds will carry a yellow-bearing and a round-bearing chromosome, or a yellow-bearing and a w^rinkled-bearing chromosome, or a greenbearing and a round-bearing chromosome, or a green-bearing and a wrinkled-bearing chromosome.
Fig. 36. Diagram to illustrate the distribution of the chromosomes in a cross between a red and a white four o'clock (see fig. 17). The chromosomes that carry the gene or factor for red are here black, and those that carry the gene for white are white.
Fig. 37. Diagram to illustrate the distribution of two pairs of chromosomes carrying two pairs of Mendelian factors, namely yellow-green and round-wrinkled. The chromosome carrying the gene for yellow is a black rod, that for green is a white circle; that carrying the gene for round is a circle with a dot, that for wrinkled is a circle without the dot.
Only these four kinds of germ-cells are possible on the chromosome mechanism. Self-fertilization of such a hybrid will give the same recombinations of chromosomes that Mendel's second law requires for the hereditary elements.
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