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End Show Slide 1 of 25 Copyright Pearson Prentice Hall 14–2 Human Chromosomes 14-2 Human Chromosomes
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End Show 14–2 Human Chromosomes Slide 2 of 25 Copyright Pearson Prentice Hall X-Chromosome Inactivation In female cells, one X chromosome is randomly switched off. This chromosome forms a dense region in the nucleus known as a Barr body. Barr bodies are generally not found in males because their single X chromosome is still active.
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End Show 14–2 Human Chromosomes Slide 3 of 25 Copyright Pearson Prentice Hall Chromosomal Disorders Sometimes things go wrong during meiosis when homologous chromosomes fail to separate.
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End Show 14–2 Human Chromosomes Slide 4 of 25 Copyright Pearson Prentice Hall Chromosomal Disorders This is known as nondisjunction, which means, “not coming apart.”
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End Show 14–2 Human Chromosomes Slide 5 of 25 Copyright Pearson Prentice Hall Chromosomal Disorders If nondisjunction occurs, abnormal numbers of chromosomes may find their way into gametes, and a disorder of chromosome numbers may result.
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End Show 14–2 Human Chromosomes Slide 6 of 25 Copyright Pearson Prentice Hall Chromosomal Disorders Nondisjunction: Occurs during Meiosis I. Homologous chromosomes fail to separate. Meiosis I: Nondisjunction Meiosis II
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End Show 14–2 Human Chromosomes Slide 7 of 25 Copyright Pearson Prentice Hall Chromosomal Disorders Down Syndrome If two copies of an autosomal chromosome fail to separate during meiosis, an individual may be born with three copies of a chromosome. Down syndrome involves three copies (AKA trisomy) of chromosome 21.
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End Show 14–2 Human Chromosomes Slide 8 of 25 Copyright Pearson Prentice Hall Chromosomal Disorders Down syndrome produces mild to severe mental retardation. It is characterized by: increased susceptibility to many diseases higher frequency of some birth defects Down Syndrome Karyotype
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End Show 14–2 Human Chromosomes Slide 9 of 25 Copyright Pearson Prentice Hall Chromosomal Disorders Sex Chromosome Disorders In females, nondisjunction can lead to Turner’s syndrome. A female with Turner’s syndrome usually inherits only one X chromosome (karyotype 45,X). Women with Turner’s syndrome are sterile.
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End Show 14–2 Human Chromosomes Slide 10 of 25 Copyright Pearson Prentice Hall Chromosomal Disorders In males, nondisjunction causes Klinefelter’s syndrome (karyotype 47,XXY). The extra X chromosome interferes with meiosis and usually prevents these individuals from reproducing.
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