PPP_Chapter 8 - CHAPTER 8 Chromosome Mutations: Variation...

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CHAPTER 8 Chromosome Mutations: Variation  in Chromosome Number and  Arrangement Dr. Sanghamitra Mohanty
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MAIN AREAS OF DISCUSSION: Terminologies describing variations in  chromosome number Monosomy Trisomy Polyploidy Variations due to chromosomal mutations Deletion Duplication Inversion Translocations Fragile sites in chromosomes
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Although most diploid species normally contain precisely two haploid chromosome sets, there are many known variations: a change in the total number of chromosomes the deletion or duplication of genes or segments of a chromosome rearrangements of the genetic material either within or among chromosomes
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Such changes are called chromosome mutations or chromosome aberrations . This is to distinguish them from gene mutations. Variations in chromosome number are known as aneuploidy when an organism gains or loses one or more chromosomes and has other than an exact multiple of the haploid set ( Table 8.1 ).
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In euploidy , complete haploid sets of chromosomes are present. Polyploidy occurs when more than two sets of chromosomes are present.
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Chromosomal variation can arise from nondisjunction , in which chromosomes or chromatids fail to disjoin and move to opposite poles during meiosis I or II ( Figure 8.1 ).
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The loss of one chromosome to produce a 2 n – 1 complement is called monosomy . Although monosomy for the X chromosome occurs in humans, monosomy for any of the autosomes is usually not tolerated in humans and other animals.
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Trisomy (2 n + 1 chromosomes) for the sex chromosomes has a less dramatic phenotype than trisomies for autosomes, which are often lethal. In trisomy, three copies of one chromosome are
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PPP_Chapter 8 - CHAPTER 8 Chromosome Mutations: Variation...

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