There was an increase of recessive lethals coupled with nonconvergence of chromosomes (sex linked) in the fruit fly.
Chromosomal aberrations can thus be monitored under unusual and abnormal conditions such as weightlessness and radiation, since chromosomes are very sensitive to stress situations.
Structural changes in blood cell chromosomes can indicate the degree of radiation damage as well as damage resulting from various environmental stresses.
Chromosomes exhibit anomalies in leukemia and mental retardation as well as in other states.
This statement concerning the minuteness of the chromosomes is not speculation--they can readily be seen and measured with the aid of the microscope.
Man has probably 16 chromosomes in the somatic cell, and the mature male and female germ cells in man contribute eight chromosomes each to the nucleus of the impregnated ovum.
After its birth she feeds it from her breasts, before birth through its umbilical vessels, but she originally gives only the eight chromosomes as the father does, and the child's vital principle builds up the body from this foundation.
The chromosomes transmit the physical bases of heredity from one generation to the next, and the heritages from the two parents are equal except in cases of prepotency.
The chromosomes then become shorter and thicker while the nucleus is completely dissolved in the protoplasm of the cell, and its membrane disappears (Fig.
Each of the chromosomes then divides into two parallel halves of equal thickness (Fig.
Directly afterwards the chromosomes arrange themselves regularly in line, like soldiers at drill, following one of the larger diameters of the cell, and forming a barrier between the two centrosomes (Fig.
The most remarkable thing, perhaps, is the almost mathematical division of the chromosomes into two halves, a division which results in the equal distribution of their substance through the whole organism.
From this moment the nuclear liquid concentrates itself around each of the groups of chromosomes, the rays disappear and the cell divides into two halves, each containing a group of chromosomes (Fig.
The nucleus of the egg and that of the spermatozoid then begin at the same time to concentrate their chromatin in the form of chromosomes (Fig.
The chromosomes then form a new meshwork of nuclear chromatin, and we have then two cells each with a nucleus and a centrosome like the mother cell (Fig.
It usually contains no chromosomes and often too little chromatin, so that it perishes when conjugation does not occur.
It forms half the number of chromosomes corresponding to the cell of the species to which it belongs, and grows at the expense of the vitellus of the egg.
Thus around each centrosome are grouped as many chromosomes as the mother cell possessed itself (Fig.
The subject is still being actively pursued, and hopes are entertained that some relation may be found between the behaviour of the chromosomes and the facts of heredity.
Biologie, 4) of the reduction of the number of chromosomes in the nucleus of both male and female gametes, and of the fact that the male and female pronuclei contribute the same number of chromosomes to the zygote-nucleus.
The twenty-four chromosomes arrange themselves at the centre of this spindle and split longitudinally, so that forty-eight chromosomes are formed.
At the next nuclear division twenty-four chromosomes appear in each nucleus, each of which divides longitudinally; and so in all subsequent divisions.
In Echinus, for instance, the two pronuclei fuse, and the spindle and chromosomes are formed from the zygote nucleus, whereas in the mouse the two pronuclei retain their distinctness during the formation of the chromosomes.
The number of chromosomes of the nucleus of the gamete is therefore reduced, and the divisions by which the gametes arise from the progametes are called reducing (maiotic) divisions.
The chromosomes usually are not readily seen in the nucleus except when the cell, along with its nucleus, is dividing.
Chromosomes from cells in the root tip of the Tradescantia plant were labeled with ³H-thymidine.
Note the duplicate sets of chromosomes moving to opposite poles of the cell.
During metaphase the chromosomes line up in one plane near the cell equator.
Today autoradiography is sufficiently precise to locate radioactively labeled substances in individual cells and even in chromosomes and other structures within the cell.
During telophase the chromosomes uncoil and return to invisibility; a new nucleus, nucleolus, and nuclear membrane are reconstituted at each end, and division of the cell body occurs between the new nuclei, forming the two new cells.
Figure 145 is a prophase showing the bivalent chromosomes still connected by linin fibers.
The scheme also assumes either selective fertilization or, what amounts to the same thing, infertility of gametic unions where like sex chromosomesare present.
The chromosomes in the spermatogonial plates were in most cases much tangled, and the behavior of the heterochromosome pair was such as to suggest an "accessory chromosome" rather than an unequal pair.
This pair divides in advance of the others, and the larger and smaller elements are plainly seen nearer the poles in anaphase than the other univalent chromosomes (figs.
The paired microchromosomes, idiochromosomes, and heterotropic chromosomes in Hemiptera.
Assuming this to be the case, a pair of small chromosomes might be subtracted from the unequal pair, leaving an odd chromosome.
It shows the same conditions as Trirhabda and Tenebrio, so far as the unequal pair of chromosomes is concerned, and is especially favorable for study of synapsis stages.
This extra chromosome is the one that contains the determiner for femaleness; each of the chromosomes of pair 24 in females contains this determiner also.
Evidently the person in whom this combination was present would differ from one all of whose chromosomes contained large-letter determiners, since part of his traits would be established by small-letter determiners.
Since we have supposed the large letters to be dominant over the small, both will have the same appearance, dependent on the presence in their chromosomes of A, B, and C.
This pairing is an essential part of the arrangement, for every hereditary bodily feature actually has two determiners governing it, which lie in corresponding positions in the two chromosomes of the pair.
There are several more things in heredity that must be taken up while we are on the subject, so we shall have to return to the chromosomes for a while.
In the ordinary cells of the germinal tissue, as we have already seen, the chromosomes are exactly like those in the other cells of the body.
The result of this is to leave the resulting cells with only half as many chromosomes as the other cells of the body have.
We saw a moment ago that thechromosomes are in pairs.
It has been proven by complicated studies which we cannot take time to describe that the determiners are grouped in the different chromosomes in a definite plan.
But whatever their number, the chromosomes are always exactly bisected before the cell divides, one-half being apportioned to each of the two cells resulting from the division.
These chromosomes vary in number in the cells of different animals, but the number is always the same for any given species of animal.
The chromosomeswithin the nucleus, we remember, are believed to be the bearers of the hereditary qualities handed down from parent to child.
These chromosomes in a given plant or animal are always constant in number.
If the chromosomes carry the determiners of the characters which are inheritable, then it is easy to see that a fertilized egg must contain an equal number of chromosomes from the bodies of each parent.
In this way the matter in the chromosomes is divided equally between the two new cells.
The chromosomes also divide, each splitting lengthwise and the parts going in equal numbers to each of the two cells formed from the old cell.
In preparing for the process of fertilization, half of these elements have been eliminated, so that when the egg and sperm cell are united they will have the full number of chromosomes that the other cells have.
These chromosomes are supposed to be the bearers of the qualities which we believe can be handed down from plant to plant and from animal to animal, in other words, the inheritable qualities which make the offspring like its parents.
But it has been found that certain cells of the body, the egg and the sperm cells, before uniting contain only half as many chromosomes as do the body cells.
The usual result of this "maturation," as it is called, is that the number of chromosomes in the ovum is reduced to a half of the normal number characteristic of the cells of the species to which it belongs.
But the suggestion that chromosomes may be stimulating, change-exciting elements, does not, Spencer goes on to say, conflict with the conclusion that the chromosomes are the vehicles conveying hereditary traits.
The spore of a fern or a moss has only half the number of chromosomes that the cells of its producer have, yet it proceeds by asexual multiplication or ordinary cell-division to build up the gametophyte or sexual generation.
As the cells which become spermatozoa are left with half the number of chromosomes possessed by preceding cells, there is actually that impoverishment and declining vigour here suggested as the antecedent of fertilisation.
Regarding the nucleus as a tiny globe, we may say that the chromosomes lie in the equatorial plane, while the two parts of the centrosome move towards the North and South Poles respectively.
The chromosomes range themselves in a belt across the centre of the nucleus, and the centrosome breaks into two parts, which take up a position one at each end of the nucleus.
The originating cell, as we have seen, has eight of its sixteen chromosomes from one parent and eight from another.
The two nuclei come into contact and coalesce, and we have thus a new cell with its sixteen chromosomes complete.
There are always a definite and invariable number of chromosomes for every species of plant or animal—the cell of a man has so many,[37] of a grasshopper so many, of a lily so many.
Other observers have in recent years demonstrated a similar relation in other genera between the number of chromosomes in the nuclei of the two generations.
Blackman, who also succeeded in showing that the nuclei of the sporophyte generation contain twice as many chromosomes as the nuclei of the gametophyte.
Studies of chromosomes of some anuran amphibians (Hylidae and Centrolenidae).
The chromosomes fuse in pairs (Conjugation of the Chromosomes, Boveri, 1892).
At the second division the diads are resolved into their constituent parts, and the "univalent" chromosomes are distributed to the daughter cells (reducing division).
This conclusion is strongly suggested, not only by the evidence in favour of the individuality of the chromosomes considered above, but also by the independent reproductive activity of the chromatin granules in the prophase of mitosis.
Not only does the "accessory chromosome" in this insect form a resting nucleus independent, and obviously physiologically differentiated from that formed from the remaining chromosomes (fig.
In sharp contrast to the pseudomitotic method is the "Eumitotic" method, in which the chromosomes are longitudinally divided at both divisions.
The difficulty is, however, at once overcome if the unripe chromosomes are associated in pairs in the equatorial plate, for the bivalent chromosomes so produced are bipolar just as are the adult (i.
It is the characteristic appearance these looped chromosomes give to the first maturation division in many Vertebrates, and especially in the Amphibia (fig.
The metaphase is the parting of the sister chromosomes in the equatorial plate; their passage to opposite poles of the spindle constitutes the anaphase; and their reconstruction to form the resting daughter nuclei, the telophase.
Moreover, the arrangement of the chromosomes must follow one of three well-marked groupings, and this is determined for each individual in the cleavage spindle of the egg and maintained throughout later development (fig.
The sister chromosomes now pass to opposite poles of the spindle (fig.
The chromosomes now arrange themselves in the "equatorial plate" of the spindle and each splits longitudinally into two[19] (fig.
As the distinct chromosomes come into view in the prophase of mitosis, two are seen to be nearly twice as long as the others, but of equal length (figs.
In the prophase of the spindle, in rare cases, some of the chromosomes are longitudinally split and transversely constricted, forming tetrads (fig.
As in the spermatogonia, 19 large chromosomes and 1 small one were found.
The dividing cells of the follicles of young eggs seemed to afford the most favorable material, but even here there was so much overlapping of the ends of the chromosomes that it was impossible to be absolutely certain of the number.
As the chromosomes go toward the poles the cell-body begins to constrict at the equator.
All the physical characteristics in a human being that come to him from his parents and remoter ancestors are supposed, by the biologists, to reach him through the chromosomes in the nuclei of the single parental germ-cells.
Wilson gives the number[30] of specific chromosomesfor seventy-four animals and plants.
When the nuclei of these two cells unite in fertilization the resulting primordial cell will have the twenty-four chromosomes restored, the specific number for this plant or animal.
The number of chromosomes in the human cell is said to be forty-eight.
There are, according to some observers, forty-seven chromosomesin man and forty-eight in woman.
Germ-cells as differentiated from the somatic cells have in the perfected cell always half the number ofchromosomes found in a somatic cell.
Whatever be the number of chromosomes that enter a new nucleus as it forms, the same number issues from it in mitosis.
If the body-cell has, say, twenty-four chromosomes, the spermatozoon of the animal or plant from which the cells are taken will have twelve chromosomes and the ovum will have twelve.
The maternal physical heredity is handed on through the chromosomes in the ovum.
Chromosomes at the poles forming the Diaster, beginning splitting of the Cell-body.
Attention was drawn to the fact that during the reducing division of nuclei which contain chromosomes of unequal size, gemini are constantly produced by the pairing of chromosomes of the same size.
When the separation of these chromosomes and their distribution to both daughter-nuclei occur a chromosome of each kind is provided for each of these nuclei.
The constancy of the numbers in which the chromosomes separate themselves from the nuclear network during division gave rise to the conception that, in a certain degree, chromosomes possess individuality.
Observers who were actively engaged in this branch of recent histological research soon noticed that the chromosomes of a given organism are differentiated in definite numbers from the nuclear network in the course of division.
The fact that in nuclei containing chromosomes of various sizes, the chromosomes which pair together in reduction-division are always of equal size, constitutes a further and more important proof of their qualitative difference.
Moreover at each stage in division chromosomes with the same differences in size reappear.
The ultimate hereditary units may, like the chromosomes themselves, retain a definite position in the resting nucleus.
This is supported also by ingenious experiments which led to an unequal distribution of chromosomes in the products of division of a sea-urchin's egg, with the result that a difference was induced in their further development.
The changes which the daughter-chromosomes undergo in the process of producing the daughter-nuclei repeat in the reverse order the changes which they went through in the course of their progressive differentiation from the mother-nucleus.
An attempt to analyze the constitution of the chromosomes on the basis of sex-linked inheritance in Drosophila.
When the homologous chromosomes come together at synapsis it has been demonstrated, in some forms at least, that they twist about each other so that one chromosome comes to lie now on the one side now on the other of its partner.
Both the cytological and the genetic evidence shows that when two X chromosomes are present a female is produced, when one, a male.
We call those classes that arise through interchange between the chromosomes "cross-over classes" or merely "cross-overs.
The production of these exceptions (primary exceptions) by a normal XX female must be due to an aberrant reduction division at which the two X chromosomes fail to disjoin from each other.
The daughter gets one of her sex chromosomes from her mother and the other from her father.
The behavior of the chromosomes as studied through linkage.
Non-disjunction of the sex-chromosomes of Drosophila.
The X chromosomes undergo crossing-over in about 60 per cent of the cases, and the crossing-over may occur at any point along the chromosome.
Clearly an interchange has taken place between the two X chromosomes in the female in such a way that a piece of one chromosome has been exchanged for the homologous piece of the other.
In the case of non-disjunction, to be given later, there is direct experimental evidence of such a nature that there can no longer be any doubt that the X chromosomes are the carriers of certain gens that we speak of as sex-linked.
While the genetic evidence forces one to accept crossing-over between the sex chromosomes in the female, that evidence gives no clue as to how such a process is brought about.
All the evidence indicates that these chromosomes carry the "factors" in inheritance which produces the characters or characteristics of the individual body.
Of the 24 chromosomes in each sperm or egg we are here concerned with only one, known as the sex chromosome because, in addition to transmitting other characteristics, it determines the sex of the new individual.
With the discovery of the place of the chromosomes in inheritance, biologists began to give their almost undivided attention to a rigid laboratory examination of the cell.
In mitosis or ordinary cell division, these chromosomes split lengthwise, so that the new cells always have the same number as the original one.
Each of these divides again by mitosis (the chromosomes splitting lengthwise), the half or haploid number remaining.
Chromosomes Chromosomes of all six species of Smilisca were studied by means of the propriono-orcein squash technique described by Duellman and Cole (1965).
Unless some arrangement was made to prevent it, the act of fertilization would cause the number of chromosomesin the fertilized ovum to be double the number characteristic of the species.
In man, for example, the addition of twenty-four chromosomes from the spermatozoon to an ovum that already contained twenty-four chromosomes of its own would mean that after fertilization the ovum contained forty-eight.
When a cell divides into two, each of these chromosomes splits by a longitudinal fissure into two halves, which appear to be exactly alike.
In the sexual act the male and the female chromosomes join forces and then the normal number is again made up, each parent contributing exactly one half.
When a cell is about to divide into two, these chromosomes become disjoined and can then be counted, and it is found that each cell of each species of animal or plant has a fixed number of these chromosomes.
Experiments of Delage and Loeb Biologists, with a few exceptions, seem to be agreed that these chromosomes are the carriers of all that which one generation inherits from another.
Thus the mouse and the lily have twenty-four chromosomes in each cell, while the ox is said to have sixteen of them per cell.
These chromosomes appear, under ordinary circumstances, to be joined together end to end, and then look like a rope in a tangle.
In the cell division, which takes place immediately before the male gamete or generative cell meets the female gamete, the chromosomes do not divide into equal halves, as is usually the case.
The nucleus of the sperm cell is supposed to contain twelve chromosomes which go through a formal rearrangement and mingling with the corresponding chromosomes in the egg cell.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "chromosomes" in a variety of sentences. We hope that you will now be able to make sentences using this word.
