Human Genetics

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Human Genetics
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• The study of human genetics requires alternative
  methods
• Studies of pedigrees are problematic because
  human families are small

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Karyotype

• Karyotyping is the analysis of chromosomes
• Cytogenetics is the study of chromosomes and
  inheritance
• Cytogenetics is based on studies of humans as
  well as Drosophila and other organisms

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Preparing a karyotype

• Harvested cells are first cltured
• The cells are then treated with colchicine which
  arrests the cells in metaphase, and then treated
  and stained to observe the chromosomes
• Chromosomes can be photographed or visualized
  using a computer, and then analyzed
• Chromosomes are identified by size, position of
  the centromere, and banding and staining regions

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Why Analyse Chromosomes and Genes?Genetic
errors arise from deletions or insertions of
genetic material, abnormal numbers of whole
chromosomes or genes, and even from misplacement
of a single base in the DNA sequence. Genetic
abnormalities can range from relatively harmless
to severe from vitamin deficiencies and food
allergies to cancer, birth defects and infant
mortality.
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Chromosomal abnormalities

• Autosome chromosomal abnormalities
• Sex chromosomal abnormalities
• Both could be affected in the structure or the
  number of the chromosomes

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• Abnormalities in chromosome number cause certain
  human disorders
• Polyploidy
• Aneuploidy

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Polyploidy

• Polyploidy is the presence of additional sets of
  chromosomes
• Polyploidy is common in plants
• Polyploidy is typically lethal in animals
• Polyploidy is due to nondisjunction or multiple
  fertilizations of an egg

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Aneuploidy

• B. Aneuploidy is a condition of an extra
  chromosome
• Extra chromosome trisomic for a particular
  chromosome
• Or a missing chromosome haploid for that
  chromosome
• Aneuploidies result from nondisjunction during
  meiosis or mitosis

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Down syndrome

• Persons with Down syndrome are usually trisomic
  for chromosome 21
• Down syndrome is a relatively common chromosomal
  abnormality
• Morphological characteristics include differences
  in the face, tongue, and hands, and individuals
  are mentally retarded
• Down syndrome is typically caused by a trisomy of
  chromosome 21, usually caused by nondisjunction
  during oogenesis

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• 4. Down syndrome is much more common in babies
  with older mothers, perhaps because an older
  woman is less likely to spontaneously abort a
  Down fetus
• 5. Affected individuals may vary in the severity
  of their symptoms
• 6. Other trisomies are typically inviable, as are
  monosomies

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Sex chromosomal abnormalties

• Most sex chromosome aneuploidies are less severe
  than autosome abnormalities

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Klinefelter syndrome

• Individuals with Klinefelter syndrome have an XXY
  karyotype
• Klinefelter males have small testes and are often
  sterile
• Klinefelter males are often tall and have
  female-like breast development about half are
  mentally impaired

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Turner syndrome

• Persons with Turner syndrome have only one X
  chromosome and no Y chromosome
• Turner syndrome females are designated as XO, and
  have undeveloped reproductive structures and are
  sterile
• No Barr bodies are apparent in persons with
  Turner syndrome

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XYY Disorders

• Some essentially normal males have an XYY
  karyotype
• Persons with an extra Y chromosome are fertile,
  phenotypically male, and often are very tall and
  have acne

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• Aneuploidies usually result in prenatal death
• Chromosome abnormalities are relatively common at
  conception but usually result in a miscarriage
• Approximately 20 of all pregnancies may be
  spontaneously aborted, often due to major
  chromosomal abnormalities

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Abnormalities in chromosome structure cause
certain disorders

• Translocation
• Translocation is attachment of part of a
  chromosome to a nonhomologous chromosome
• This is the cause of approximately 4 of Down
  syndrome

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• Deletion
• A deletion is loss of part of a chromosome
• Most large deletions are nonviable
• An example of a deletion is the cri-du-chat
  syndrome

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• Fragile sites
• Fragile sites are weak points at specific sites
  in chromatids
• Fragile sites appear to be held together by thin
  strands of chromatin
• Fragile X syndrome is a relatively common cause
  of mental retardation, particularly in males

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