Chromosomal Mutations:Variation in Chromosome Number

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Chromosomal MutationsVariation in Chromosome
Number

• Chapter 8 Sections 8.1 8.7

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Chromosomal Mutations

• Deletion part of the chromosome is missing
• Starts with breaks in the chromosome
• Radiation, heat, viruses, chemicals, etc.
• May cause an unpaired loop
• May give rise to pseudodominance
• Cri-du-chat syndrome (Chrom. 5)

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Chromosomal Mutations

• Duplication doubling of a segment of a
  chromosome
• Tandem
• Reverse tandem
• Terminal tandem
• Position Effect of barring eyes in Drosophila

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Chromosomal Mutations

• Inversion results when a segment of a
  chromosome is excised and then reintegrated in an
  orientation 180 degrees from the original
  orientation.
• Pericentric inversion
• Paracentric inversion
• Resulting from a dicentric bridge

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Chromosomal Mutations

• Translocation change in position of chromosome
  segments and the gene sequences they contain to a
  different location
• Nonreciprocal intrachromosomal
• Nonreciprocal interchromaosomal
• Reciprocal interchromosomal

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Chromosomal Mutations

• Fragile Sites and Fragile X Syndrome
• Narrowings or unstained areas (gaps)
• Recessive X-linked traits (predominately males)
• Normal transmitting males carrying a premutation
  will have grandsons with mental retardations and
  some granddaughters (up to 33) with mild
  retardation

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Variation in Chromosomal Number

• Euploidy the correct number of sets of
  chromosomes in an organism
• Monoploidy only one set of chromosomes when
  there should be more
• Male wasps, ants, and bees are monoploids because
  they develop from unfertilized eggs
• Used in plant-breeding experiments/colchicine
  (inhibits mitotic spindle)

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Variation in Chromosomal Number

• Polyploidy having three or more sets of
  chromosomes
• Usually occurs due to a breakdown of the mitotic
  spindle
• Usually occurs in plants (self-fertilization)
• Even numbered polyploids have a better chance of
  being fertile than odd numbered polyploids
• In humans, triploidy is usually lethal.

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Variation in Chromosomal Number

• Polyploidy (continued)
• Autopolyploidy
• All sets of chromosomes originate in the same
  species/defect in meiosis
• Example a diploid gamete fuses with a haploid
  gamete to produce a triploid zygote (bananas and
  seedless fruit)

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Variation in Chromosomal Number

• Polyploidy (continued)
• Allopolyploidy
• Sets of chromosomes originate from different
  species though usually related
• Because of differences between chromosomes, the
  hybrid, no crossing over occurs and no viable
  gametes produced making hybrids sterile
• Occasionally, tow sets of different chromosomes
  will double, producing tissues of 2N 2N2 /
  produces fully fertile allotetraploid, 2N 2N2,
  plants

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Variation in Chromosomal Number

• Aneuploidy variation in the number of
  chromosomes
• Nullisomy - loss of one homologous chromosome
  pair 2N 2
• Monosomy loss of a single chromosome
• 2N 1
• Trisomy single extra chromosome 2N 1
• Tetrasomy extra chromosome pair 2N 2

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Variation in Chromosomal Number

• In humans, Down Syndrome is the only disorder
  where an extra autosomal chromosome exists and
  the individuals lives past three months. All
  other embryos will self-abort or the infant will
  die within 6 months. However, extra sex
  chromosomes will not cause an embryo to
  necessarily abort but the individual may have
  other distinguishable characteristics.

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Variation in Chromosomal Number

• Trisomy-21 Down Syndrome
• Trisomy-13 (Patau syndrome) cleft lip and
  palate, small eyes, polydactyly and mental
  retardation. Dies within 3 months
• Trisomy-18 (Edwards syndrome) developmental
  retardation, congenital malformations, and die
  within 6 months

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