Karyotyping

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Karyotyping

• E. McIntyre
• Biology 12

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Amniocentesis
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Karyotyping
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• A karyotype analysis usually involves blocking
  cells in mitosis (metaphase) and staining the
  condensed chromosomes with Giemsa dye. The dye
  stains regions of chromosomes that are rich in
  the base pairs Adenine (A) and Thymine (T)
  producing a dark band.
• A common misconception is that bands represent
  single genes, but in fact the thinnest bands
  contain over a million base pairs and potentially
  hundreds of genes.
• The analysis involves comparing chromosomes for
  their length, the placement of centromeres (areas
  where the two chromatids are joined), and the
  location and sizes of G-bands.

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Just for Laughs...

• The "Y-chromosome" is the human male sex
  chromosome. This illustration indicates some of
  the genes proposed to be located on the
  chromosome. Obviously, there will be multiple
  versions of each gene, i.e. different alleles
  which, in different males, would result in
  different phenotypes (different expressions of
  traits). Can you think of other genes that are
  likely to be associated with this chromosome?

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Chromosomal Nondisjunction

• Chromosomal Nondisjunction is the failure of
• chromosome pairs to separate properly during cell
  
• division. There are two possible causes
• 1. a failure of homologous chromosomes to
  separate in meiosis I
• 2. the failure of sister chromatids to separate
  during meiosis II.

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Chromosomal Disorders
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Why is too many chromosomes problem?

• The genes on the chromosomes are needed in
  exactly the correct amount in order to work
  correctly.
• Many genes cause the cell to make proteins of
  various kinds, and these are needed in the
  correct amounts. Some genes function by
  instructing other genes to turn on and off at the
  right moment, and these instructions need to be
  precise, accurate and sensitive.
• Having an extra chromosome is like having a
  business with too many managers or a car with two
  drivers or a country with two presidents, perhaps
  giving contradictory instructions.

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Turner Syndrome Monosomy of X Chromosome (X0)
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Cri du Chat

• Deletion of part of upper arm of Chromosome 5

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Patau's SyndromeTrisomy of Chromosome 13

• Extra fingers or toes (polydactyl)
• Deformed feet, known as rocker-bottom feet
• Neurological problems such as small head
  (microcephaly), failure of the brain to divide
  into halves during gestation (holoprosencephaly),
  severe mental deficiency
• Facial defects such as small eyes
  (microphthalmia), absent or malformed nose, cleft
  lip and/or cleft palate
• Heart defects (80 of individuals)
• Kidney defects

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Edward's SyndromeTrisomy of Chromosome 18

• lethal for most babies (death before 1 yr of age)
  
• major physical abnormalities and severe mental
  retardation

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Down SyndromeTrisomy 21

• mild to severe form of mental retardation
  accompanied by distinctive physical traits.
  People with Down syndrome have an irregularity
  with autosome pair 21. In most cases, there is
  an extra chromosome (i.e., trisomy).
• Down syndrome individuals typically have short,
  stocky bodies with thick hands and feet.
• Their hands commonly have a simian crease
• broad, short heads with small low-set ears, small
  concave saddle-shaped or flattened noses,
  relatively large ridged tongues that roll over a
  protruding lower lip, low muscle tone, and loose
  joints.

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