Reproduction. Types of sexual reproduction Sexual reproduction irregular forms of sexual reproduction
Various types of sexual reproduction include parthenogenetic, gynogenetic and androgenetic reproduction of animals and plants.
The listed types of sexual reproduction arose as a result of the complete or partial loss of meiosis and its replacement by mitosis in the cycle of sexual reproduction. Asexual reproduction in this case is secondary.
We have already pointed out that in most species of animals and plants, during sexual reproduction, the fusion of two gametes occurs - male and female. In a number of animal and plant species, virgin reproduction occurs without the participation of sperm. The development of an embryo from an unfertilized egg is called parthenogenesis.
Parthenogenesis is divided into natural and artificial. During natural parthenogenesis, an egg that has or has not undergone division of maturation, under the influence of internal or external causes, begins to fragment and develops into a normal embryo without any participation of the sperm. The phenomenon of natural parthenogenesis is characteristic of lower crustaceans, rotifers, hymenoptera (bees, wasps), etc. It is also known in birds (turkeys).
Parthenogenesis can be permanent (obligate parthenogenesis) or partial (facultative parthenogenesis). In some animals, only females can develop from unfertilized eggs, and males from fertilized ones, in others - both sexes, in others - only males develop from unfertilized eggs, and females from fertilized eggs.
Parthenogenetic reproduction can alternate in generations with sexual reproduction (cyclic parthenogenesis). In lower crustaceans (Daphnia), as well as in aphids, rotifers and other animals, a change of generations is observed, developing from normally fertilized and from parthenogenetic eggs. In Daphnia, females are diploid, males are haploid. Under favorable external conditions, daphnia reproduce parthenogenetically. In this case, only one sex appears - female, since the eggs do not undergo meiotic division. The reason for this phenomenon will become clear when the genetic and cytological mechanisms of sex determination are considered. The onset of unfavorable external conditions, such as a drop in temperature or lack of food, leads to the fact that females begin to lay haploid eggs. From these eggs males develop parthenogenetically. After mating and normal fertilization, the sexual generation is restored female organisms with a diploid number of chromosomes. Fertilized eggs in cysts can survive wintering and unfavorable external conditions. A similar picture is observed in grass aphids and other insects.
A distinction is made between somatic, or diploid, and generative, or haploid, parthenogenesis. During somatic parthenogenesis, the egg does not undergo reduction division, or, if it does, two haploid nuclei fuse together and restore the diploid set of chromosomes (autokaryogamy); Thus, the diploid set of chromosomes is preserved in the tissue cells of the embryo. In some cases, somatic cells of such organisms may have an increased number of chromosomes due to non-disjunction of entire sets of chromosomes. In generative parthenogenesis, the embryo develops from a haploid egg. As a rule, male individuals (bees, scale insects, mites) develop from such eggs. For example, in the honey bee, drones develop from unfertilized haploid eggs through parthenogenesis. During the development of germ cells, meiosis is replaced by mitosis, and therefore sperm have a haploid set of chromosomes. In contrast to the embryonic path, the diploid set of chromosomes can be restored in the soma of such animals.
Artificial parthenogenesis is the experimentally induced activation of unfertilized eggs. The honor of this discovery belongs to the Russian zoologist A. A. Tikhomirov, who first carried out artificial parthenogenesis in 1885 on eggs silkworm. Artificial parthenogenesis can be caused by high temperature, acids, light and other agents. The possibility of artificial parthenogenesis has been proven for many aquatic and terrestrial invertebrates (sea urchins, stars, insects, etc.) and vertebrates (amphibians).
With artificial parthenogenesis, the normal development of the embryo is often inhibited. However, using an improved technique for treating silkworm eggs with high temperature at a certain exposure and at a certain stage of development, B. L. Astaurov managed to obtain large numbers of female parthenogenetic butterflies. Currently, artificial parthenogenesis has also been carried out in frogs and rabbits. Artificial parthenogenesis has also been obtained in plants (algae, fungi and higher plants: cereals, legumes, etc.). It is stimulated by irritation of the stigma with foreign or killed pollen, as well as talc, chalk, etc. In this case, as in animals with parthenogenetic development, inheritance occurs only through the maternal line.
The type of parthenogenetic reproduction can also include gynogenetic reproduction, i.e., the development of the embryo exclusively due to the female nucleus. Unlike parthenogenesis, in this case the participation of a sperm is necessary to stimulate the development of the egg (pseudogamy), but fertilization (karyogamy) does not occur in this case. Gynogenesis has been found in hermaphroditic roundworms, the viviparous fish Mollienisia formosa, and in silver crucian carp, which is found in our Far East. Gynogenesis, as a rule, is found in individuals of a species at the boundaries of its range as a mechanism that guarantees the conservation of the species there.
The gynogenetic development of eggs can be artificially induced if the sperm is irradiated with X-rays, treated with chemicals or exposed to high temperature before fertilization. In this case, the sperm nucleus is destroyed, and it loses the ability for karyogamy, but such a sperm can activate the egg. Natural and artificial gynogenesis also occurs in plants and is caused by the same factors as in animals. In the case of natural gynogenesis, developing individuals contain a normal diploid number of chromosomes. Artificial gynogenesis is often associated with haploidy, so such embryos have little viability.
The phenomenon of parthenogenetic and gynogenetic reproduction has great importance for the study of heredity, since in this case the offspring are completely similar to the maternal organism. The study of parthenogenesis, like gynogenesis, is also important for solving a number of practical issues, in particular for obtaining individuals of one specific sex from some practically important objects, for example, for breeding valuable breeds of fish and chickens.
Sometimes, under artificial conditions, when the female nucleus with part of the cytoplasm is removed, the remaining part of the egg, after penetration of the sperm into it, begins to fragment, which soon stops. This experimentally induced initial development of the embryo from part of the cytoplasm of the egg without the participation of the female nucleus is called merogenesis. In some cases (in sea urchin) egg crushing can be achieved in the complete absence of the nucleus. Nuclear-free parthenogenetic merogones developed into abortive morulae or blastulae, which soon died.
The reproduction of plants and animals without fertilization is called apomixis with fertilization (karyogamy) - amphimixis. Some researchers (S.S. Khokhlov and others) put a broader content into the concept of apomixis - any asexual reproduction, including all its types. The terms apomixis and amphimixis apply equally to plants and animals.
But since apomixis is especially widespread in flora and is of great importance for the study of inheritance, let us consider its features. The types of apomixis in plants are extremely diverse, but there is still no generally accepted classification for them.
The most common type is the parthenogenetic formation of an embryo from an egg. In this case, in the case of diploid parthenogenesis, meiosis completely disappears, and in the case of haploid parthenogenesis, normal megasporogenesis proceeds and a haploid embryo sac is formed with a reduced set of chromosomes in the nuclei. In diploid parthenogenesis, the constancy of the number of chromosomes in a double set is maintained by mitosis, and the inheritance of the characteristics of the endosperm and embryo occurs only through the maternal line.
In haploid parthenogenesis, the embryo is formed from a haploid egg, also without fertilization. The plants developing from such an embryo are sterile, weak and small-leaved. This plant can only reproduce vegetatively. In this case, for the formation of full-fledged seeds, pseudogamy is necessary - activation of the embryo sac by the pollen tube. One sperm from the tube, reaching the embryo sac, is destroyed, and the other merges with the central nucleus and participates only in the formation of endosperm tissue (species of cinquefoil - Potent ilia, raspberry - Rubus, etc.). Inheritance will work somewhat differently here. The characteristics of the embryo and plant are inherited only through the maternal line, while the characteristics of the endosperm can appear both paternally and maternally.
The emergence of an embryo not from an egg, but from other cells of the female gametophyte (synergids and antipodes), which have and have not undergone meiosis, is called apogametic. In addition to these two types of apomixis, there are others, but they are of more specialized interest, in particular for plant embryologists.
All types of apomictic reproduction (excluding vegetative) apparently arose as a result of various simplifications of the normal sexual process. But no matter how diverse the types of apomixis are, if meiosis is omitted, it is replaced by mitosis, and fertilization is completely excluded. With some types of apomixis, meiosis and reduction in the number of chromosomes are preserved in the embryo sac, then fertilization is replaced by autogamy,
that is, the development of the embryo is carried out due to the fusion of the haploid nuclei of the embryo sac itself. The latter is necessary to maintain the constancy of the species number of chromosomes. With any type of apomixis, the principle of constancy of the number of chromosomes of a species cannot be violated, since otherwise the number of chromosomes in cells will either increase or decrease with each generation. Knowledge of the type of apomixis is absolutely necessary for studying genetic phenomena in individuals that reproduce in this way. An example of this is the episode that took place in the experiments of G. Mendel.
Mendel, wanting to find out the universality of the patterns of inheritance he discovered in peas, conducted many years of experiments with one of the species of the genus Hieracium. But apomictic reproduction is widespread in this plant. By crossing different races of hawkweeds, Mendel did not receive confirmation of the desired results precisely due to the lack of fertilization in this object. This circumstance was one of the reasons that prevented Mendel from extending the laws he discovered to other organisms.
Natural parthenogenesis and apomixis play an important role in evolution as special mechanisms that ensure hereditary diversity.
Currently, genetic techniques are being developed using apomixis in practical purposes to consolidate heterosis, maintain valuable mutations, etc.
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Division characteristic first of all single-celled organisms. As a rule, this is done by simple division cells in two. Some protozoa(for example, foraminifera) division occurs into a larger number of cells. In all cases, the resulting cells are completely identical to the original one. The extreme simplicity of this method of reproduction, associated with the relative simplicity of organization single-celled organisms, allows you to reproduce very quickly. So, in favorable conditions The number of bacteria can double every 30-60 minutes. An organism that reproduces asexually is capable of endlessly reproducing itself until a spontaneous change in the genetic material occurs - mutation. If this mutation is favorable, it will persist in the progeny of the mutated cell, which will represent a new cell clone.Reproduction by spores
Asexual reproduction of bacteria is often preceded by the formation of spores. Bacterial disputes- these are resting cells with reduced metabolism, surrounded by a multilayer membrane, resistant to desiccation and other unfavorable conditions that cause the death of ordinary cells. Sporulation serves both to survive such conditions and to spread bacteria: once in a suitable environment, the spore germinates, turning into a vegetative (dividing) cell.Asexual reproduction with the help of unicellular spores is also characteristic of various mushrooms And algae. Disputes in many cases arise through mitosis(mitospores), and sometimes (especially in fungi) in huge quantities; upon germination, they reproduce the mother's organism. Some fungi, such as the harmful plant pest Phytophthora, form motile spores equipped with flagella, called zoospores or wanderers. After floating in droplets of moisture for some time, such a wanderer “calms down”, loses its flagella, becomes covered with a dense shell and then, under favorable conditions, germinates.
In addition to mitospores, many of these organisms, as well as all higher plants, form spores of another kind, namely meiospores, formed by meiosis. They contain the haploid set chromosomes and give rise to a generation, usually not similar to the maternal one and reproducing sexually. Thus, the formation of meiospores is associated with alternation of generations- asexual (giving spores) and sexual.
Vegetative propagation. Another option for asexual reproduction is carried out by separating from the body a part of it, consisting of a larger or smaller number of cells. From them the adult organism develops. An example would be budding in sponges And coelenterates or reproduction plants shoots, cuttings, bulbs or tubers. This form of asexual reproduction is usually called vegetative reproduction. It is fundamentally similar to the regeneration process. Plays an important role in plant growing practices. Thus, it may happen that a sown plant (for example, an apple tree) has some successful combination of characteristics. At the seeds of this plant this successful combination will almost certainly be disrupted, since seeds are formed as a result of sexual reproduction, and this is associated with gene recombination. Therefore, when growing apple trees, vegetative propagation is usually used - by layering, cuttings or grafting buds onto other trees.
Asexual reproduction, which reproduces individuals identical to the original organism, does not contribute to the emergence of organisms with new variants of characteristics, and thereby limits the ability of species to adapt to new environmental conditions. The means to overcome this limitation was the transition to sexual reproduction.
Budding
n Some species of unicellular organisms are characterized by such a form of asexual reproduction as budding. In this case, mitotic division of the nucleus occurs. One of the formed nuclei moves into the emerging local protrusion of the mother cells, and then this fragment buds off. The daughter cell is significantly smaller than the mother cell, and it requires some time to grow and complete the missing structures, after which it acquires the appearance characteristic of a mature organism. Budding is a type vegetative propagation. Many lower animals reproduce by budding mushrooms, For example yeast and even multicellular animals, for example freshwater hydra. When yeast budding, a thickening forms on the cell, which gradually turns into a full-fledged daughter yeast cell. On the body of the hydra, several cells begin to divide, and gradually a small hydra grows on the mother individual, which forms a mouth with tentacles and intestinal cavity, associated with the intestinal cavity of the “mother”. If the maternal individual catches prey, then some of the nutrients enter the small hydra, and vice versa, the daughter individual, while hunting, also shares food with the maternal individual. Soon the small hydra separates from the mother’s body and is usually located next to her. (But not always!)
Body division. Some organisms can reproduce by dividing the body into several parts, and from each part a full-fledged organism grows, similar in all respects to the parent individual (flat and annelids, echinoderms).
87. Sexual reproduction. Regular and irregular forms.
Sexual reproduction- process for the majority eukaryotes associated with the development of new organisms from germ cells(in unicellular eukaryotes with conjugation The functions of germ cells are performed by the sex nuclei).
The formation of germ cells is usually associated with the passage meiosis at some stage life cycle body. In most cases, sexual reproduction is accompanied by the fusion of germ cells, or gametes, in this case, a doubled set of chromosomes, relative to gametes, is restored. Depending on the systematic position of eukaryotic organisms, sexual reproduction has its own characteristics, but as a rule, it allows the genetic material from two parent organisms to be combined and produces offspring with a combination of properties not found in the parental forms.
The effectiveness of combining genetic material in descendants obtained as a result of sexual reproduction is facilitated by:
chance meeting of two gametes;
random arrangement and divergence to the division poles of homologous chromosomes during meiosis;
crossing over between chromatids.
In many groups of eukaryotes, the secondary disappearance of sexual reproduction has occurred, or it occurs very rarely. In particular, to the department deuteromycetes(fungi) unites a wide group of phylogenetic ascomycetes And basidiomycetes who have lost the sexual process. Until 1888, it was assumed that among terrestrial higher plants sexual reproduction is completely lost in sugar cane. The loss of sexual reproduction has not been described in any group of metazoans. However, many species are known (lower crustaceans - daphnia, some types worms), capable of reproducing under favorable conditions parthenogenetically over tens and hundreds of generations. For example, some types rotifers for millions of years they reproduce only parthenogenetically, even while forming new species (!).
In a number of polypliodic organisms with an odd number of sets of chromosomes, sexual reproduction plays a small role in maintaining genetic variability in the population due to the formation of unbalanced sets of chromosomes in gametes and descendants.
The ability to combine genetic material during sexual reproduction is of great importance for the selection of model and economically important organisms.
88. Cytological bases of sexual reproduction. Meiosis is a specific process in the formation of germ cells.
Meiosis(from Greek meiosis - decrease) or reduction division cells - nuclear division eukaryotic cells with decreasing number chromosomes twice. Occurs in two stages (reduction and equational stages of meiosis). Meiosis should not be confused with gametogenesis- education of specialized germ cells, or gametes, from undifferentiated stem.
With a decrease in the number of chromosomes as a result of meiosis in life cycle there is a transition from the diploid phase to the haploid phase. Recovery ploidy(transition from haploid to diploid phase) occurs as a result sexual process.
Due to the fact that in the prophase of the first, reduction stage, pairwise fusion occurs ( conjugation) homologous chromosomes, the correct course of meiosis is possible only in diploid cells or in even polyploids (tetra-, hexaploid, etc. cells). Meiosis can also occur in odd polyploids (tri-, pentaploid, etc. cells), but in them, due to the inability to ensure pairwise fusion of chromosomes in prophase I, chromosome divergence occurs with disturbances that jeopardize the viability of the cell or developing from it a multicellular haploid organism.
The same mechanism underlies the sterility of interspecific hybrids. Since interspecific hybrids combine in the cell nucleus the chromosomes of parents belonging to various types, chromosomes usually cannot enter into conjugation. This leads to disturbances in chromosome segregation during meiosis and, ultimately, to the non-viability of germ cells, or gametes. Certain restrictions on the conjugation of chromosomes are imposed by chromosomal mutations(large deletions, duplications, inversions or translocations).
Meiosis consists of 2 consecutive divisions with a short interphase between them.
Prophase I- prophase of the first division is very complex and consists of 5 stages:
Leptotene or leptonema- packaging of chromosomes.
Zygotene or zygonema- conjugation (connection) of homologous chromosomes with the formation of structures consisting of two connected chromosomes, called tetrads or bivalents.
Pachytena or pachynema - crossing over(crossover), exchange of sections between homologous chromosomes; homologous chromosomes remain connected to each other.
Diplotena or diplonema- partial decondensation of chromosomes occurs, while part of the genome can work, the processes of transcription (RNA formation), translation (protein synthesis) occur; homologous chromosomes remain connected to each other. In some animals, in oocytes, chromosomes at this stage of meiotic prophase acquire a characteristic shape chromosomes like lamp brushes.
Diakinesis- DNA condenses to the maximum again, synthetic processes stop, the nuclear membrane dissolves; The centrioles diverge towards the poles; homologous chromosomes remain connected to each other.
Metaphase I- bivalent chromosomes line up along the equator of the cell.
Anaphase I- microtubules contract, bivalents divide and chromosomes move towards the poles. It is important to note that, due to the conjugation of chromosomes in zygotene, whole chromosomes, consisting of two chromatids each, diverge to the poles, and not individual chromatids, as in mitosis.
Telophase I
Prophase II- condensation of chromosomes occurs, the cell center divides and the products of its division disperse to the poles of the nucleus, the nuclear membrane is destroyed, and a fission spindle is formed.
Metaphase II- univalent chromosomes (consisting of two chromatids each) are located at the “equator” (at an equal distance from the “poles” of the nucleus) in the same plane, forming the so-called metaphase plate.
Anaphase II- univalents divide and chromatids move towards the poles.
Telophase II- chromosomes despiral and a nuclear envelope appears.
89.Gametogenesis. The structure of germ cells.
Gametogenesis divided into spermatogenesis (the process of sperm formation in males) and oogenesis (process of egg formation). In terms of what happens to DNA, these processes are practically the same: one initial diploid cell gives rise to four haploid ones. However, in terms of what happens to the cytoplasm, these processes are radically different.
Accumulate in the egg nutrients, necessary for the further development of the embryo, therefore the egg is a very large cell, and when it divides, the goal is to preserve nutrients for the future embryo, therefore the division of the cytoplasm is asymmetrical. In order to preserve all the reserves of the cytoplasm and at the same time get rid of unnecessary genetic material, polar bodies are separated from the cytoplasm, which contain very little cytoplasm, but allow the division of the chromosome set. Polar bodies are separated during the first and second meiotic divisions
90. Patterns of spermatogenesis in mammals and humans.
91. Regularities of oogenesis in mammals and humans.
The first phase of the wound process is inflammatory phase- characterized by traumatic tissue swelling, increased vascular permeability, acidosis, migration of leukocytes, mast cells and macrophages. The wound is cleansed by phagocytosis and lysis of necrotic tissue.
In the second phase of the wound process - regeneration phase- granulation tissue develops, gradually filling the wound defect. The main structures of this tissue are fibroblasts, intercellular substance and capillaries. Fibroblastic differential cells of granulation tissue differ from fibroblasts of normal connective tissue in their high functional activity. They synthesize proteins and glycosaminoglancans, forming collagen fibers. Macrophages, mast cells and plasma cells also play an important role in the development and maturation of granulation tissue. Granulation tissue subsequently transforms into scar connective tissue.
The third phase of the wound process - scar reorganization phase- characterized by a progressive decrease in the number blood vessels and cellular elements (fibroblasts, macrophages, mast cells) with the phenomena of an increase in the total mass of collagen fibers. In parallel with the maturation of granulation tissue and its transformation into scar tissue, epithelization of the wound occurs. Epithelization of the wound and maturation of granulation tissue strictly correspond in time.
Depending on the nature and magnitude of the injury, features of the body's reactivity and other conditions, the wound process proceeds differently. With a small volume of damage, wound healing occurs by primary intention. Inflammation and replacement of defects in tissues follow directly from their traumatic swelling and are not accompanied by suppuration. By the end of the first week, the wound process is largely complete. If the volume of the lesion is large and the edges of the wound are at a more or less significant distance from each other, then wound healing occurs through suppuration with the formation of well-developed granulation tissue, followed by scarring. Wound healing occurs by secondary intention with a significant duration of the phases of the wound process.
Wound healing by primary and secondary intention has quantitative, but not qualitative differences. The mechanisms of regeneration are fundamentally similar and include inflammation, proliferation of connective tissue, and epithelization. Knowledge of the key parts of the regeneration process allows us to purposefully search for means of regulating wound healing and developing methods of tissue therapy.
In animals and plants, so-called irregular types of sexual reproduction occur. This is, first of all, apomixis (from the Greek “apo” - without, “mixis” - mixing), i.e. sexual reproduction without fertilization. Apomixis is the opposite of amphimixis (“amphi” - divided), i.e., sexual reproduction that occurs through the fusion of gametes of different quality. A synonym for apomixis is parthenogenesis, i.e. virgin reproduction from the Greek. "parthenos" - virgin). The term apomixis is more often used in relation to plants, and parthenogenesis - in relation to animals.
Along with parthenogenesis, egg development is also observed, activated by sperm not involved in fertilization. The male pronucleus dies, and the body develops at the expense of the female pronucleus. This phenomenon is called gynogenesis, which occurs in hermaphrodite roundworms and some fish.
The opposite of gynogenesis is androgenesis - development only due to the male pronucleus in the event of the death of the female pronucleus. Haploid androgenesis is very rare. The development of androgenic individuals to adulthood was observed only in the Habrobracon ichneumon wasp and the silkworm.
In the silkworm, during fertilization, several sperm penetrate the egg, but the nucleus of only one of them merges with the nucleus of the egg, the rest die. If unfertilized eggs are activated by temperature shock, as described above, and irradiated with X-rays, the nucleus of the egg will die. If such enucleated eggs are further inseminated, then the two male pronuclei that have penetrated the egg merge with each other. Due to the resulting diploid nucleus, a zygote develops. As shown by B. JI. Astaurov, such androgenetic zygotes always turn into males, since they carry two identical sex chromosomes - ZZ. Obtaining purely male offspring from silkworms is economically beneficial, since males are more productive than females.
TO irregular types sexual reproduction can include:
- parthenogenetic,
- gynogenetic,
- androgenetic
reproduction of animals and plants.
Parthenogenesis is the development of an embryo from an unfertilized egg. The phenomenon of natural parthenogenesis is characteristic of lower crustaceans, rotifers, hymenoptera (bees, wasps), etc. It is also known in birds (turkeys). Parthenogenesis can be stimulated artificially by causing activation of unfertilized eggs through exposure to various agents. Parthenogenesis is distinguished:
- somatic, or diploid,
- generative, or haploid.
At somatic in parthenogenesis, the egg does not undergo reduction division or, if it does, two haploid nuclei fuse together and restore the diploid set of chromosomes (autokaryogamy); Thus, the diploid set of chromosomes is preserved in the tissue cells of the embryo. At generative In parthenogenesis, the embryo develops from a haploid egg. For example, in the honey bee (Apis mellifera), drones develop from unfertilized haploid eggs through parthenogenesis.
Gynogenesis . Gynogenetic reproduction is very similar to parthenogenesis. Unlike parthenogenesis, gynogenesis involves spermatozoa as stimulators of egg development(pseudogamy), but fertilization (karyogamy) does not occur in this case; the development of the embryo is carried out exclusively due to female core. Gynogenesis has been found in roundworms, the viviparous fish Molliensia formosa, in silver crucian carp (Platypoecilus) and in some plants - buttercup (Ranunculus auricomus), bluegrass (genus Poa pratensis), etc. Gynogenetic development can be caused artificially, if sperm or pollen is irradiated with X-rays, treated with chemicals or exposed to high temperatures before fertilization. In this case, the nucleus of the male gamete is destroyed and the ability for karyogamy is lost, but the ability to activate the egg is retained.
The phenomenon of gynogenetic reproduction is of great importance for the study heredity, since in this case the offspring receives hereditary information only from mothers. Thus, with asexual reproduction, parthenogenesis and gynogenesis, the offspring should be similar only to the maternal organism.
Androgenesis . The direct opposite of gynogenesis is androgenesis. During androgenesis, egg development occurs only due to male nuclei and maternal cytoplasm. Androgenesis can occur in cases where the maternal nucleus dies for some reason before fertilization. If one sperm enters the egg, then the developing embryo with a haploid set of chromosomes turns out to be non-viable or poorly viable. Viability androgenic zygotes are normalized if the diploid set of chromosomes is restored.
