Nondisjunction

Nondisjunction during meiosis is the most mutual cause of the aneuploidies, which can also occasionally result from a chromosomal rearrangement (e.g., Robertsonian translocation or a balanced sex chromosome translocation) [26].

From: Man Reproductive and Prenatal Genetics , 2019

Nondisjunction

J. Forejt , in Encyclopedia of Genetics, 2001

Nondisjunction in Translocation Heterozygotes

Organisms heterozygous for a reciprocal chromosome translocation are prone to higher frequency of abnormal meiotic disjunction, including nondisjunction. Only alternative disjunction, combining either both translocated chromosomes or both intact homologs in the gamete, leads to a balanced, euploid genome. Next I and adjacent 2 disjunctions combine i translocated and i intact chromosome from the pachytene translocation cross and event in partial nullisomy of i translocated chromosome associated with partial disomy of the other chromosome involved in the rearrangement ( Figure two). Nondisjunction can occur also in the 3:1 form when three or one chromosomes involved in the translocation cross (Effigy 2) enter the secondary gametocyte. The resulting Due north+ane aneuploid gamete contains an extra chromosome composed of 2 chromosomes involved in the translocation. Afterward fertilization with a normal gamete, this extra chromosome gives rising to tertiary trisomy of the embryo. The Due north−2 aneuploid gamete, if functional, results in preimplanation lethality when fused with an euploid gamete.

Figure 2. Chromosome disjunction in meiosis of a reciprocal translocation heterozygote. Only alternant (Alt.) disjunction results in balanced gametes. Recombination between a centromere and the translocation intermission (non shown in the picture) results in uneven chromatids, 1 of which can end in unbalanced, adjacent I production and the other can yield a counterbalanced gamete with alternative disjunction. Nondisjunction (3:1) tin can pb to tertiary trisomies.

The unbalanced gametes occur in translocation heterozygotes with a frequency of approximately 50% and effect in inviable embryos. The phenomenon is referred to equally 'semisterility' in mice, since the translocation heterozygotes brandish nigh half of the normal number of pups in their litters. Human being reciprocal translocation carriers can have a family history of frequent spontaneous abortions.

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Genetic Disorders Affecting Growth and Development

Robert S. Sparkes , Barbara F. Crandall , in Fetal-Placental Disorders, 1972

i NONDISJUNCTION

Nondisjunction means that a pair of homologous chromosomes has failed to separate or segregate at anaphase then that both chromosomes of the pair pass to the same daughter cell. This probably occurs virtually usually in meiosis, but it may occur in mitosis to produce a mosaic private. The cause(s) of nondisjunction is non known; the following are some possibilities. Abortuses and neonates with trisomy 21 and with trisomy 18 are associated with increasing maternal age, suggesting the mother'south age may be an important etiological factor. Structural abnormalities of the chromosomes such as translocations and pericentric inversions may interfere with chromosome pairing at meiosis and promote nondisjunction. Some families seem to have an inherited tendency for nondisjunction; for case, they may have several children with trisomy 21 or they may have a child with Klinefelter'due south syndrome and some other with Down's syndrome.

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Cytogenetics in Reproduction

Cynthia C. Morton , Charles Lee , in Yen & Jaffe'due south Reproductive Endocrinology (Sixth Edition), 2009

Meiotic Nondisjunction

Meiotic nondisjunction errors are common in humans, resulting in aneuploidy, a term used when the total number of chromosomes in a jail cell is not an verbal multiple of the haploid number. Aneuploidy ordinarily involves a single chromosome, just in rare circumstances, may involve more than 1. Aneuploidy is nowadays in approximately 0.6% of newborns 56 and nearly lxx% of spontaneous abortions. 57 Trisomy for all chromosomes has been observed in spontaneous abortions, indicating that nondisjunction for each chromosome does occur. 58-sixty

Meiotic nondisjunction can involve only one chromosome or the whole chromosome fix. Nondisjunction of a unmarried chromosome will produce germ cells that accept either 2 (disomy) or zero (nullisomy) copies of the specific chromosome. If a germ prison cell with an extra chromosome is combined with a chromosomally normal germ cell, the product will be trisomic (i.due east., having 47 chromosomes). If a germ jail cell missing a chromosome is combined with a chromosomally normal germ cell, the product will exist monosomic (i.e., having 45 chromosomes). Nondisjunction of the entire chromosome set will pb to either germ cells with 2 copies of every chromosome or germ cells with no chromosomes. The clinical phenotype and the histopathology of conceptuses that tin result from numerically abnormal gametes depend on both the total number of chromosomes and the relative number of paternal versus maternal chromosomes.

Nondisjunction can take place in either meiosis I or meiosis Ii. If nondisjunction occurs in meiosis I, all four products of meiosis will be chromosomally abnormal. Two of the four products of meiosis will have 2 copies of the chromosome involved in the nondisjunction outcome, and two of the four products of meiosis will have no copies of that particular chromosome. Of farther note, in germ cells with 2 copies of the chromosome, the copies, although homologous, will non be identical. Homologous chromosomes practise not separate in nondisjunction errors in meiosis I, only sister chromatids split up properly in meiosis Ii. Thus, each of the germ cells with an extra chromosome will have a maternally derived chromosome and a paternally derived chromosome. In the absence of recombination, one chromosome would be entirely of maternal origin and the other entirely paternal.

If nondisjunction occurs in meiosis Two, two of the four products volition be unaffected by the event and two of the products will be abnormal. Ane abnormal product will have an extra chromosome, and the other aberrant product will be missing that chromosome. With nondisjunction errors in meiosis Ii, homologous chromosomes separate properly in meiosis I, but sister chromatids do not split in meiosis Two. Thus, in dissimilarity to meiosis I nondisjunction errors, the ii nondisjoined chromosomes would be genetically identical in the absence of recombination (Fig. 31-7). This apparently little difference between errors in meiosis I and errors in meiosis Two tin have important clinical consequences that are discussed later. Furthermore, the written report of the parental origin of chromosomes involved in aneuploidies has led to of import observations nearly the origin and meiotic stage of nondisjunction.

It is difficult to study meiotic nondisjunction for all chromosomes directly in gametes and even indirectly in products of conception because many aneuploid products of conception are lost early in pregnancy and are never brought to clinical attention. Nonetheless, conceptuses with trisomies for some chromosomes survive long enough to be clinically recognized, and several studies have used Deoxyribonucleic acid polymorphisms to analyze the parental origin of the extra chromosome in these cases of trisomy. These studies have shown that maternal nondisjunction accounts for significantly more cases of autosomal trisomy than does paternal nondisjunction. For trisomies 13, fourteen, 15, 16, xviii, 21, and 22, maternal nondisjunction accounted for 88%, 83%, 88%, 100%, 93%, 91%, and 89% of cases, respectively. 54,61-65 Direct studies of aneuploidy in gametes give estimates of 2% to 4% meiotic nondisjunction in sperm 58,66,67 and 13% to 18% meiotic I nondisjunction in oocytes. 68,69 These findings signal that the excess of maternal nondisjunction relative to paternal nondisjunction seen in trisomic conceptuses does indeed reverberate differences in the charge per unit of nondisjunction in oocytes and spermatocytes. It is still possible, however, that selection against aneuploid sperm occurs afterwards spermatogenesis likewise.

The rate of aneuploidy is higher in oocytes than in spermatocytes, and it likewise increases markedly with maternal (just not paternal) age. 69,70 Although the relationship between increased maternal age and Down syndrome was first described in 1933, 71 the machinery for this aging event remains to exist fully elucidated. Most nondisjunction errors in oocytes occur in meiosis I, and it has been hypothesized that the prolonged arrest in meiosis I contributes to these errors. 72

Paternal nondisjunction is more than common in cases of aneuploidy involving sexual activity chromosomes than in cases involving autosomes. Information technology has been hypothesized that the XY bivalent is more susceptible to nondisjunction than are the homologous bivalents. FISH studies on sperm have supported this hypothesis, showing rates of sex chromosome disomy to be two to 4 times higher than disomy for particular autosomes. 66,73 Eighty percent of 45,X karyotypes can be attributed to paternal nondisjunction, although some of these cases may exist acquired past early loss of the Y chromosome through mitotic nondisjunction in the zygote. 74,75 Cases of 47,XXY are divided approximately as betwixt maternal and paternal nondisjunction. 76,77 Yet, as with the autosomal trisomies, the extra Ten chromosome is of maternal origin in xc% of cases of 47,30. 77

Studies have shown that recombination is crucial for proper segregation of homologous chromosomes. Yeast experiments have shown that recombination is required for formation of the synaptonemal complex and for complete pairing of homologous chromosomes. From this, it has been suggested that, in the absence of pairing and recombination, nondisjunction would exist increased. 78 In humans, studies of trisomies 15, 16, eighteen, and 21, too every bit of XXY and XXX, take shown that, on average, the particular chromosomes involved in a specific nondisjunction consequence participated in fewer recombinations than usual. 54,79-83 Presumably, the express region of homology in which recombination occurs in the XY bivalent accounts for its increased susceptibility to nondisjunction. Interestingly, the overall rate of recombination is higher in female person gametogenesis than in male person gametogenesis, although some specific chromosomal regions, including the telomeric regions of many chromosomes, accept higher recombination rates in males. 84,85

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The In Vitro Micronucleus Assay

Ann Doherty , ... Jeffrey C. Bemis , in Genetic Toxicology Testing, 2016

6.iii.4 Nondisjunction Assay

Nondisjunction can be examined in binucleate human being lymphocytes past using centromere-specific probes to examine distribution of pairs of chromosomes between the two nuclei [13].

Cytochalasin B blocks cells at cytokinesis, leading to an accumulation of binucleate cells. The incorporation of centromere-specific probes allows visualization of chromosome segregation and distribution in the private nuclei of the binucleate cell. When using two centromere-specific probes, the normal distribution of chromosomes would be two copies in each nucleus written equally a 2:2 distribution. Nondisjunction is the malsegregation of chromosomes due to the failure of chromosomes on the metaphase plate to split to each girl nuclei and may be determined by a iii:1 or four:0 distribution of centromere-specific signals (Figure 6.3).

Figure vi.iii. Cartoon of cell partition with nighttime and calorie-free small circles representing individual centromere-specific probes: (A) 2:2 normal distribution of chromosomes to daughter nuclei; (B) 3:1 nondisjunction of the chromosome represented by a calorie-free circle; (C) iv:0 nondisjunction of the chromosome represented by a light circle; (D) both a 3:1 nondisjunction of the chromosome represented by a light circumvolve and four:0 nondisjunction of the chromosome represented by a dark circle; and (Eastward) both a three:one nondisjunction of the chromosome represented by a night circle and loss of one copy of the chromosome represented by a light circle in a micronuclei.

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Trisomy X Syndrome (47,XXX)☆

K. Wigby , ... R. Wilson , in Reference Module in Biomedical Sciences, 2018

Genetic Counseling Recommendations

Nondisjunction is typically a random occurrence. Trisomy X is not unremarkably inherited, and recurrence run a risk for parents of one child with trisomy X is estimated to be less than 1% for future pregnancies. However, the risk for any aneuploidy must consider maternal age.

The recurrence adventure for a woman with trisomy 10 to have a child with trisomy X or 47,XXY has been demonstrated to be one–v%. In addition, women with trisomy X should be counseled that while fertility is typically normal, there is an increased risk for POF, and advisable guidance should be provided.

When counseling families on the prognosis, management, and treatment of trisomy Ten, information technology is important to emphasize the significant variability of this condition. While the incidence is estimated to be 1/m past newborn screening, ~   90% of these cases are estimated to go undiagnosed. In add-on, it is of import to circumspection about inaccurate and biased information on the internet and other outdated sources. Resources listed at the end of this article should be provided for all new diagnoses for additional information and support.

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Environmental Effects on Development—Teratology

James G. Wilson , in Fetal-Placental Disorders, 1972

two CHROMOSOMAL NONDISJUNCTION

Chromosomal nondisjunction also mainly affects germ cells, but does and then during the meiotic divisions of the maturation procedure, rather than at all times, as is likely to exist the case with mutations. Mosaic chromosomal conditions, nevertheless, probably take origin from mitotic errors in early cleavage stages of embryogenesis when chromosomal pairs in dividing blastomeres fail to segregate one to each girl cell. Regardless of when information technology occurs, the issue of chromosomal aberration is visible backlog or deficiency in the chromosomes, or parts thereof, in the cells of the offspring arising from the affected germinal or cleavage cells. Deficiency of chromosomal material is poorly tolerated, usually being lethal if involving more than one arm of an autosome, although absence of an entire X chromosome is tolerated in Turner's syndrome with relatively minor adverse effects on development. Excess of chromosomes is but slightly less detrimental. Trisomy of some of the smaller autosomal pairs is compatible with survival beyond term only results in severe developmental defects. On the other hand, excesses of either sexual practice chromosome crusade little or no phenotypic change in afflicted individuals. The causes of chromosomal nondisjunction are not entirely understood but in that location is substantial prove that aged germ cells are more likely to exhibit nondisjunction than younger ones, especially as regards germ cells from older parents ( 163 , 164 ), and possibly also regarding germ cells that take part in fertilization afterward prolonged residence in the genital tract ( 234 ).

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Genetic Programming in Ovarian Development and Oogenesis

JOE LEIGH SIMPSON , in Handling of the Postmenopausal Adult female (Tertiary Edition), 2007

Four Ten CHROMOSOMAL MOSAICISM: 45,X/46,XX AND 45,X/47,Thirty

If nondisjunction or anaphase lag occurs in the zygote and embryo, two or more cell lines may result (mosaicism) ( Fig. iii.2). The final complement volition depend on the stage at which aberrant prison cell sectionalization occurs and on the types of daughter cells that survive following nondisjunction or anaphase lag. Detection of mosaicism depends on the number of cells analyzed per tissue and on the number of tissues analyzed (sixteen, 17).

FIGURE 3.2. Diagrammatic representation of the products of (A) normal mitosis and (B) mitosis characterized past nondisjunction of a Y chromosome. If all daughter cells survived, the complement would be 45,X/46,XY/47,XYY.

 From ref. 17.

The well-nigh common form of mosaicism associated with gonadal dysgenesis is 45,X/46,XX. Individuals with a 45,10/ 46,Twenty complement predictably bear witness fewer anomalies than do 45,X individuals. Simpson (sixteen) tabulated that 12% of 45,X/46,XX individuals menstruate, compared with only three% of 45,Ten individuals. Among 45,Ten/46,Twenty individuals, 18% undergo chest development, compared with 5% of 45,X individuals. Mean developed tiptop is greater with a 45,10/46,20 complement than with 45,X; more mosaic (25%) than nonmosaic (5%) patients reach adult heights greater than 152 cm (16). Somatic anomalies are less probable to occur in 45,X/46,20 than in 45,X.

In Sybert'south review and analysis (31), spontaneous menstruation occurred in 45% to 57% (depending on whether mode of ascertainment was in her clinic or from published reports, respectively); frequency of brusque stature (below the third percentile) was 45% (v/11) and 87% (vii/eight); fertility occurred in xiv% (one/vii) and 69% (ix/13).

Less mutual but phenotypically similar to 45,10/46,Twenty individuals is 45,10/47,XXX. 45,X/47,XXX occurs less frequently but is phenotypically similar to 45,X/46,XX. Individuals with 45,10/46,XY may also testify bilateral streak gonads; even so, more frequently they show a unilateral streak gonad and a contralateral dysgenetic testis (mixed gonadal dysgenesis).

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SEX CHROMOSOMES IN DROSOPHILA

Ursula Mittwoch , in Sexual practice Chromosomes, 1967

Iii Nondisjunction of the X-chromosome

The miracle of nondisjunction of the Ten-chromosome in Drosophila is of particular interest, beginning because it provides straight proof that the sexual activity-linked genes are borne on the 10-chromosome; and second because information technology is now known that it occurs in man and is responsible for abnormalities in sexual development.

In the typical grade of sex activity-linked inheritance a recessive factor carried by the female parent gives rise to affected sons and unaffected daughters, whereas a sex-linked gene borne by the father is transmitted to his daughters and to none of his sons. Occasionally in Drosophila this criss-cantankerous blueprint is broken: Some infrequent daughters bear their mother'south sex-linked character, while the sons resemble the father.

Bridges (1913, 1914, 1916) plant that on mating a female person with normal center color to a white-eyed male, about 5% of the daughters were like the mother and about 5% of the sons were similar the begetter. He explained this anomaly on the supposition that a small proportion of the mother's sex chromosomes failed to disjoin at meiosis, thus producing some eggs with two X-chromosomes and some with none (Fig. 4.five). Nondisjunction of the sexual activity chromosomes had previously been observed cytologically by Wilson (1909d) in the spermatocytes of Metapodius and Frolowa (1912) had plant exceptional eggs containing two Ten-chromosomes or no X-chromosome in Parascaris equorum. The correctness of the assumption that the exceptional daughters were of XXY-chromosome constitution was confirmed by analysis of the chromosomes in their oogonia (Bridges, 1916). The formation of XXX-zygotes was assumed on hypothetical grounds; such individuals were not really constitute until several years after (Bridges, 1921).

FIG. 4.five. Nondisjunction in Drosophila.

The incidence of nondisjunction in ordinary stocks of Drosophila is of the guild of one in 2000 individuals, but among the offspring of not-disjunctional mothers the incidence of abnormal offspring due to secondary nondisjunction is about one in twenty (T. H. Morgan et al., 1925). Clearly, the separation of the sexual practice chromosomes in the oogenesis of an XXY-private is bound to be abnormal. Either the ii Ten-chromosomes stay together and the Y-chromosome goes to some other cell, or the Ten- and Y-chromosomes disjoin as a pair from a single X (Fig. 4.six). Bridges (1916) found that in Drosophila the latter alternative happens more than ofttimes. Fertilization by an X-begetting sperm volition give rise to Twenty- and XXY-daughters, while a Y-bearing sperm volition result in XY and XYY-sons. All these individuals volition be unexceptional as regards the inheritance of sexual activity-linked characters, since the daughters receive an X-chromosome each from their female parent and their father, while the sons receive their unmarried X from their mother. By dissimilarity, eggs containing two X-chromosomes or a single Y-chromosome will requite rise to exceptional sons and daughters. The daughters, of chromosome constitution XXY, have received both 10-chromosomes from their female parent and therefore express their female parent'southward sex-linked characters; and the sons, though of normal XY-constitution, accept received the Y-chromosome from their mother and the X-chromosome from their male parent, and thus resemble their father as regards sex activity-linked characters. In improver, two other classes of zygotes may be expected from these types of nondisjunctional eggs: those with three X-chromosomes and those with two Y-chromosomes and no X. The latter type have never been found, and it may be concluded that the presence of at least i 10-chromosome is essential for viability (this is not so in organism's in which the sexual activity chromosomes are little differentiated, e.1000., fishes, run across Chapter vii, Section 2). In Drosophila, the early development of eggs defective an X-chromosome has been institute to be very abnormal, with an oxygen consumption no higher than that of unfertilized eggs (Poulson, 1945). Individuals with three X-chromosomes were constitute (Bridges, 1921) every bit a result of nondisjunction as well equally among the offspring of triploid females. Such individuals are very inviable; the survivors are abnormal sterile females. Owing to the excessive number of 10-chromosomes which they contain they take been called "super-females" (Bridges, 1921), in spite of the fact that the ovaries are underdeveloped. Stern (1959) proposed the term "meta-female person," implying that the phenotype is across femaleness. Many authors, especially in human cytogenetics, prefer the term "triple Ten-female."

FIG. 4.vi. Secondary nondisjunction in Drosophila.

Stocks exist in Drosophila in which two X-chromosomes are joined by their centromeres; in addition, the females conduct a Y-chromosome. In such "attached-X" stocks, the two X-chromosomes always disjoin as a pair from the Y, and consequently all the offspring will be aberrant with respect to the inheritance of sexual practice-linked genes: the females receive both X-chromosomes from the mother and a Y-chromosome from the male parent, whereas the males receive their X-chromosome from the father and their Y-chromosome from their female parent.

Nondisjunction in Drosophila females was recently studied by J. R. Merriam and Frost (1964); they confirmed, from the distribution of exceptional offspring of experimental crossings, that nondisjunction, is, in fact an error of meiosis. In Drosophila, nondisjunction is not affected past maternal age (Kelsall, 1963), but an increase occurs after X-irradiation (Mavor, 1921; A. M. Clark and Clark, 1963).

Nondisjunction has been defined as master, when the female parent is of XX-sex chromosome constitution, and every bit secondary, when the mother is XXY (T. H. Morgan et al., 1925). In Drosophila, secondary nondisjunction is always a very much more than frequent event.

Master nondisjunction in the male is more difficult to detect, since information technology does not normally issue in offspring with phenotypes opposite to the rules of sexual practice-linked inheritance. That this phenomenon occurs was shown by Kelsall (1961) from the offspring of flies with a special type of chromosome constitution and containing markers both on the paternal and maternal 10-chromosomes. In add-on to the usual type of nondisjunction, this mating gave rise to XXY-females containing one paternal and one maternal X-chromosome, which must therefore have arisen from an XY sperm. In addition, XYY-males with a maternal X-chromosome were plant, which probably arose from a YY-sperm.

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Toxicology Testing and Evaluation

E. Zeiger , in Comprehensive Toxicology, 2010

3.10.ii.3 Aneuploidy

Aneuploidy, also chosen nondisjunction, is the unequal distribution of chromosomes to girl cells during jail cell partitioning. It is usually acquired by an interference with the structure or function of the mitotic spindle (made of proteins), which is responsible for separation of the chromosomes to the developing girl cells. Alternatively, it could be caused by chromosome damage so that the chromosome is not recognized or fairly 'picked up' by the mitotic spindle. Therefore, dissimilar chromosome aberrations, aneuploidy must be measured in the daughter cells, and tin can be measured as a gain or loss of a chromosome, but is more hands measured as the gain. Non all aneuploid cells will survive and reproduce.

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Mechanisms and Morphology of Cellular Injury, Adaptation, and Death1

Margaret A. Miller , James F. Zachary , in Pathologic Basis of Veterinary Disease (Sixth Edition), 2017

Errors in Prison cell Division.

Most chromosomal disorders are caused past errors in prison cell division, which transfers the disorder within somatic and/or germline cells. Abnormalities of chromosome number and/or structure tin can arise in autosomes (chromosomes in somatic cells) or sex chromosomes (germline cells). There are 2 kinds of cell segmentation, mitosis and meiosis (East-Fig. i-30). Mitosis is somatic prison cell division by which the body grows and differentiates and tissues regenerate. Mitotic division results in ii daughter cells, each with chromosomes and genes identical to those of the parent cell. In contrast, meiosis occurs simply in cells of the germline and results in the formation of ova or spermatozoa, with, nether normal conditions, each prison cell blazon having one-half of the normal karyotype (one of each kind of autosome and either an 10 or a Y chromosome).

East-Figure 1-30. Mitosis and Meiosis.

A comparing of normal mitotic (A) and meiotic (B) jail cell partitioning.

 (Courtesy Dr. K.A. Miller, College of Veterinarian Medicine, Purdue University; and Dr. J.F. Zachary, College of Veterinary Medicine, University of Illinois.)

After fertilization a unmarried-cell zygote gives rise to all cells of the body (estimated at 1 × x14 cells), which are derived from dozens or even hundreds of mitoses. The biologic significance of meiosis and mitosis lies in ensuring the constancy of chromosome number and thus the integrity of the genome from one cell to its progeny and from one generation to the side by side. The medical significance of these processes involves errors of cell division, which pb to the formation of an private prison cell or of a prison cell lineage with an abnormal number of chromosomes and thus an inappropriate amount of genomic cloth. Such errors are called nondisjunctions and represent a failure of chromosome pairs to disjoin (dissever) during cell division, and as a consequence both chromosomes go to one cell and none to the other. Meiotic nondisjunction, especially in oogenesis, is a common mutational mechanism, responsible for chromosomally abnormal fetuses. In those fetuses that survive to term, chromosome abnormalities crusade developmental defects, failure to thrive, and reduced mental function. Mitotic nondisjunctions can likewise be inherited. Nondisjunction soon afterwards fertilization, either in the developing embryo or in extraembryonic tissues similar the placenta, leads to chromosomal mosaicism that tin can be the basis for some genetic disorders. Additionally, abnormal chromosome segregation in rapidly dividing tissues can be a step in the evolution of tumors.

Numeric Alterations.

Cells with normal chromosome numbers have euploid karyotypes (i.east., normal number of chromosomes for the species). If an fault occurs in meiosis or mitosis and a cell acquires a lesser or greater number of chromosomes, the abnormal karyotype is referred to as aneuploidy. One cause of aneuploidy is nondisjunction during meiosis ( E-Fig. 1-31), resulting in either actress chromosomes (e.thousand., trisomy, tetrasomy) or ane less chromosome (i.e., monosomy) (see East-Fig. 1-31). Fertilization of such ova past normal spermatozoa results in ii types of zygotes, trisomic (or tetrasomic) or monosomic. Trisomic or tetrasomic offspring are extremely rare in domestic animals, only an autosomal trisomy has been reported in an Italian Friesian dogie with malformed limbs, congenital opisthotonus, brachygnathia, blindness, and absence of external genitalia. Monosomic offspring are more common in domestic animals, in which an X chromosome monosomy (Turner-similar syndrome) has been reported mainly in horses (see Chapter eighteen). The vulva, uterus, and ovaries in affected mares are smaller than normal; most fail to bike or prove estrous behavior.

E-Effigy 1-31. Nondisjunction.

Nondisjunction is the failure of homologous chromosomes (chromatids) to split up properly during meiotic jail cell division. A, Nondisjunction at the get-go meiotic division. B, Nondisjunction at the second meiotic sectionalization.

 (Courtesy Dr. M.A. Miller, College of Veterinarian Medicine, Purdue University; and Dr. J.F. Zachary, College of Veterinarian Medicine, University of Illinois.)

Occasionally, mitotic errors in early development requite ascension to two or more populations of cells with dissimilar chromosomal karyotypes in the same animal, a condition referred to equally mosaicism. Mosaicism can issue from mitotic errors during the division of the fertilized ovum or in somatic cells. Mosaicism affecting the sexual activity chromosomes is relatively common. In the sectionalisation of the fertilized ovum, an error may lead to one of the daughter cells receiving three sex chromosomes, whereas the other receives just one, yielding, for instance, an n-one, X/n+i, XXX mosaic. All cells derived from each of these cells volition have the same abnormal karyotype. An case of X (sex) chromosome mosaicism occurs in tortoiseshell and calico (tricolored) cats. In all female mammalian cells, the function of one X chromosome is inactivated through a random process chosen X chromosome inactivation. Approximately 50% of the cells of tricolored cats take inactivated paternal X chromosomes; the other 50% take inactivated maternal 10 chromosomes. Thus female person tricolored cats take roughly equal populations of two genetically different cell types and are therefore a type of mosaic that is expressed in the patterns of hair coloration (orange, blackness, and white).

Autosomal mosaicism seems to exist much less mutual than that involving the sexual activity chromosomes. An mistake in an early on mitotic division affecting the autosomes usually leads to a nonviable mosaic fetus.

Structural Alterations.

Changes in the construction of chromosomes are caused by deletion, inversion, duplication, or translocation of a portion of a sexual practice or autosomal chromosome during prison cell division (Eastward-Fig. 1-32). During embryogenesis, structural alterations of sex chromosomes are more common than those of autosomes and can result in some cells having XX and others having XY sexual activity chromosomes. These cells coexist, so both male and female reproductive structures develop to varying degrees dependent on the expression of the sex chromosomes. Equally a effect, these diseases are characterized by sexual ambivalence and include hermaphroditism and pseudohermaphroditism (come across Chapters eighteen and nineteen). Structural alterations also likely involve autosomes in animals, simply their occurrence and significance have not been adequately characterized.

Due east-Figure 1-32. Forms of Chromosomal Rearrangements.

Chromosomal rearrangements are chromosome abnormalities characterized by structural changes in chromosomes such as missing, extra, or irregular segments of chromosomal DNA. They are caused by breakage of Dna double helices from errors in Deoxyribonucleic acid replication and/or from damage caused by mutagens. These rearrangements include deletions, duplications, inversions, translocations, isochromosomes, and ring chromosomes.

 (Courtesy Dr. One thousand.A. Miller, College of Veterinary Medicine, Purdue Academy; and Dr. J.F. Zachary, College of Veterinarian Medicine, University of Illinois.)

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