Your Immediate Past

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Your Immediate Past

• Chapter 7

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The 23 Chromosome Pairs of Homo sapiens

• Autosomes chromosomes 1-22
• Sex chromosomes X and Y
• Autosomes carry genes for all functions
• Sex genes carry mostly genes for sex determination

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Haploid and Diploid

• Haploid one set of chromosomes
• n chromosomes
• In most animals found in sperm and eggs
• Diploid two sets of chromosomes
• 2n chromosomes
• Somatic cells
• Fertilized egg

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Meiosis

• Is a special kind of cell division
• Occurs in testes and ovaries
• Prepares cells for fertilization
• Produces sperm and eggs
• Number of chromosomes are halved
• Cells go from diploid of chromosomes to a
  haploid

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Meiosis creates unique combinations of genes

• Why you look different from your siblings
• Why your children look different from each other
• Two events in meiosis contribute to genetic
  variation
• Gene shuffling
• Chromosome shuffling

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Special Events in Meiosis

• Gene shuffling Recombination or crossing over
• This step is important for creating genetic
  variation or diversity

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Special events in Meiosis

• Chromosome Shuffling
• This step is also important for creating genetic
  variation or diversity

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• At the End of Meiosis
• ½ the normal number of chromosomes
• Genetic variation
• Gene shuffling
• Chromosome shuffling

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When Things go Wrong in Meiosis

• Numbers matter
• Ideally humans have 46 chromosomes
• Changes in whole sets of chromosomes
• Lethal in humans
• OK in plants
• Additions or deletions of ONE chromosome
• Also bad news except for Downs Syndrome
• Tolerated by the Sex chromosomes

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• Polyploidy Abnormalities in the number of
  haploid chromosomal sets.
• Example
• Triploidy Individual has three sets of
  chromosomes. FrequencyTriploidy account for 15
  18 of spontaneous abortions in humans.

• Prognosis considered a lethal condition.
• Triploid embryo usually fails to complete
  development
• Newborn usually doesnt live longer than 1 month

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Karyotype of triploid Human
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Abnormalities during meiosis changes chromosome
number
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Down Syndrome Trisomy 21

• 1 in 900 live births
• Distinctive physical appearance wide skull, flat
  in back. Skin fold in corner of eye, tongue
  furrowed protruding, abnormalities in finger
  and palm prints,
• Development Mental retardation (but large
  variations in mental ability), slow growth,
  developmental delays
• Health Impact 40 have heart defects, Prone to
  respiratory infection, High leukemia rate.

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Karyotype of a Down Syndrome Male
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Why Is Non-disjunction Usually Maternal?

• IDEAS
• Older Mother has greater chance of
  non-disjunction. ..if woman is 40 at time of
  conception, approx. 40 years to complete prophase
  I
• Perhaps uterus of older mother less sensitive to
  abmormal fetus. So older mothers less likely to
  spontaneously abort abnormal fetus.
• Or perhaps a combination of these factors.