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Slide 113.1 Nondisjunction Leads to Changes in Chromosome NumberSlide 3Chromosome NondisjunctionSlide 5Nondisjunction in Meiosis IISlide 7Slide 8Aneuploidy in HumansSlide 10Trisomy 21Turner SyndromeReduced Fertility in AneuploidyReduced Fertility in Aneuploidy, continuedMosaicismGynandromorphs13.2 Changes in Euploidy Result in Various Kinds of PolyploidyAutopolyploidy and AllopolyploidyAllopolyploidyConsequences of PolyploidyReduced Recessive HomozygosityPolyploidy and EvolutionSlide 23Slide 24Chromosome InversionChromosome TranslocationThree Types of TranslocationsCopyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated ApproachPart 3:Select Material from CH 13Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach13.1 Nondisjunction Leads to Changes in Chromosome Number2•Non-disjunction is the process of failed chromosome and sister chromatid segregation.•It can result in abnormalities in chromosome numbers •The number of chromosomes in a nucleus and the relative size and shape of each chromosome are species-specific characteristics.•The euploid number of chromosomes of an individual is the # of complete sets. (n, 2n, 3n)•If cells contain a number of chromosomes that is not euploid, chromosome number is aneupliod.Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach3Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated ApproachChromosome Nondisjunction4•Chromosome nondisjunction is the failure of homologous chromosomes or sister chromatids to separate as they normally do during cell division.•In somatic cells, it can result in one daughter cell with an extra chromosome (2n+1) and the other missing one chromosome (2n-1).• The relatively poor survival of these cells normally limits their number in animals. • The result of nondisjunction in germ-line cells is aneuploid gametes that can produce aneuploid zygotes.• Nondisjunction in meiosis I results from failure of homologs to separate; the gametes produced are either n+1 or n-1• Fusion of these gametes with normal (n) gametes produces trisomic (2n+1) or monosomic (2n-1) offspring.Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach5Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated ApproachNondisjunction in Meiosis II•The result of nondisjunction in meiosis II is the failure of sister chromatids to separate normally.•Among the four gametes produced by meiosis, only two will be affected in this case. •Two of the gametes will be normal (n); of the remaining two, on will be n+1 and the other will be n-1.6Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach7Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach8ANIMATION: NondisjunctionCopyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated ApproachAneuploidy in Humans9•Humans are enormously sensitive to gene dosage changes and aneuploids usually don’t survive.•Only trisomies of chromosomes 13, 18, and 21 are seen in newborn infants.•No autosomal monosomies are observed.•Multiple forms of sex-chromosome trisomies, and one type of sex-chromosome monosomy occur.•Trisomies and monosomies other than those found in newborn infants are known to occur in humans (adults).•Studies that monitor human pregnancies indicate that about half of all conceptions spontaneously abort during the first trimester.•About half of these pregnancies carry abnormalities of chromosome number and structure.Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach10Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated ApproachTrisomy 2111•Trisomy 21, or Down syndrome, has been the focus of intense study.•Research has identified a link between the risk of trisomy 21 and maternal age.•A small number of genes on chromosome 21 are responsible for the cognitive disabilities and heart abnormalities that are principle symptoms.•A portion of chromosome #21 called DSCR (Down syndrome critical region) can be correlated with the majority of the Down Syndrome symptoms. •A candidate gene called DYRK, known to produce dosage-sensitive learning defects in in mice and flies, makes a contribution to Down Syndrome.Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated ApproachTurner Syndrome•Turner syndrome is a monosomy of the x chromosome, with no second sex chromosome (XO).•In XO embryos, the single copy of the gene SHOX, which is not inacitivated by dosage compensation, is insufficient to direct normal development.•The haploinsufficiency of this gene plays a central role in producing the symptoms of the syndrome.12Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated ApproachReduced Fertility in Aneuploidy13•In trisomics, chromosome segregation during meiosis is disturbed because of failure to properly pair and segregate.•Two patterns of synapsis are possible: a trivalent synapsis or bivalent and univalent arrangement.•Neither mechanism can segregate three chromosomes equally at anaphase I.Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated ApproachReduced Fertility in Aneuploidy, continued•In trisomics, meiosis results in two chromosomes moving to one pole and one chromosome moving to the other.•Thus half the gametes contain two copies of the chromosome; these will produce trisomic offspring that are unlikely to survive.•This results in a form of semi-sterility, in which only some of the offspring produced are viable.14Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated ApproachMosaicism15•Mosaicism can develop as a result of mitotic non-disjunction early in embryogenesis.•For example, 25-30% of Turner syndrome cases occur in females that are mosaic, with some 45, XO cells (missing one) and some 46, XX•Some Turner syndrome individuals carry 47, XXX cells too.Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated ApproachGynandromorphs16•In some insects such as butterflies and fruit flies, sex-chromosome mosaicism produces gynandromorphs.•These are individuals with some female and some male cells.•These occur due to sex-chromosome nondisjunction in early development.Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach13.2 Changes in Euploidy Result in


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LSU BIOL 2153 - CH 13 Notes

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