Chromosomal Disorders

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Chromosomal Disorders

• Learning Target What is nondisjunction, and what
  problems does it cause?

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Section 14-2

• Human Chromosomes
• Chromosomal Disorders
• 1. Down Syndrome
• 2. Sex Chromosome Disorders

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Nondisjunction

• The most common error in meiosis occurs when
  homologous chromosomes fail to separate. This is
  known as nondisjunction.
• Nondisjunction can occur either during meiosis I,
  or in meiosis II.
• 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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Nondisjunction during Meiosis I
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Trisomy 

• If two copies of an autosomal chromosome fail to
  separate during meiosis, an individual may be
  born with three copies of a chromosome, known as
  a trisomy.
• The most common form of trisomy involves three
  copies of chromosome 21 and is called Down
  syndrome.

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Downs Syndrome

• Down syndrome produces mild to severe mental
  retardation.
• It is also characterized by an increased
  susceptibility to many diseases and a higher
  frequency of some birth defects.
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Sex Chromosome Disorders 

• Two of these abnormalities are Turners syndrome
  and Klinefelters syndrome

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Turners Syndrome

• In females, nondisjunction can lead to Turners
  syndrome.
• A female with Turners syndrome inherits only one
  X chromosome (genotype XO).
• Women with Turners syndrome are sterile because
  their sex organs do not develop at puberty.

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Klinefelters syndrome

• In males, nondisjunction causes Klinefelters
  syndrome (genotype XXY).
• The extra X chromosome interferes with meiosis
  and usually prevents these individuals from
  reproducing.
• Klinefelters syndrome have been found in which
  individuals were XXXY or XXXXY.
• No reported instances of babies being born
  without an X chromosome, indicating that the X
  chromosome contains genes that are vital for
  normal development.

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Y chromosome

• The Y chromosome contains a sex-determining
  region that is necessary to produce male sexual
  development, even in combination with several X
  chromosomes. However, if this region of the Y
  chromosome is absent, the embryo develops as a
  female.

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Assessment !!

• Distinguish between sex-linked disorders and sex
  chromosome disorders.