CYTOGENETICS

• Cytogenetics


  The study of chromosomes and their role in heredity

  
• Jennifer G. Opiso


  Researcher, IBS Plant Biology Division
• Normal karyotype/ideogram


  Visual representation of the chromosomes in a cell

  
• Numerical changes in chromosomes


  • Heteroploidy - Changes in the number of whole chromosomes
  • Euploidy - Changes in entire sets of chromosomes

  
• Haploidy


  Chromosome complement of gametes
• Monoploidy


  Has only one set of chromosomes

  
• Monoploidy


  • Male wasps, ants, bees (developed from unfertilized egg)
• Polyploidy


  Increase in the number of chromosome sets

  
• Polyploidy


  • Triploid (3n) - Seedless banana

  Tetraploid (4n) - Some wheat, cotton

  Hexaploid (6n) - Some wheat, kiwi fruit
• Parthenogenesis


  Development of an embryo from an unfertilized egg cell
• Types of parthenogenesis


  • Thalytoky - Females produced

  Deuterotoky - Both genders produced

  Arrhenotoky - Males produced
• Numerical changes in chromosomes


  • Aneuploidy - Loss or addition of a single whole chromosome

  Trisomy (2n+1)

  Monosomy (2n-1)

  Nullisomy (2n-2)

  Double trisomy (2n+1+1)

  Tetrasomy (2n+2)

  
• Humans with nullisomy (2n-2) will not survive
• Age of mother


  Risk of trisomy 21 child
• Risk of trisomy 21 child by maternal age


  • Less than 39: 1/3000

  30-34: 1/600

  35-39: 1/280

  40-44: 1/70

  45-49: 1/40

  All mothers combined: 1/665

  
• Types of chromosome deletions


  • Terminal - Deletion at the terminal portion

  Interstitial - Deletion at the internal portion
• Types of chromosome inversions


  • Pericentric - Inverted segment includes the centromere

  Paracentric - Inverted segment does not include the centromere

  
• Types of chromosome duplications


  • Tandem - ABCBCDEFGH

  Reverse - ABCCBDEFGH

  Displaced - ABCDEFGHAB

  
• Cri-du-chat syndrome


  Genetic disorder caused by partial deletion of chromosome 5
• DiGeorge syndrome


  Genetic disorder caused by deletion of a small part of chromosome 22
• Prader-Willi syndrome


  Genetic disorder caused by deletion, unbalanced translocation, or uniparental disomy of chromosome 15
• Angelman syndrome


  Genetic disorder caused by deletion or mutation of the UBE3A gene on chromosome 15
• Chromosome 15 is associated with several neurodevelopmental disorders
• 16p11.2 deletion syndrome is associated with developmental delay, intellectual disability, and autism spectrum disorders
• DYRK1A gene on chromosome 21 is associated with intellectual disability and autism
• Balanced translocations


  Structural chromosomal rearrangements where two chromosomes exchange segments

  
• Reciprocal translocations


  Part of one chromosome is exchanged with another
• Philadelphia chromosome


  Abnormal chromosome formed by translocation of parts of chromosomes 9 and 22
• Burkitt lymphoma


  Aggressive B-cell cancer characterized by translocation and deregulation of the MYC gene on chromosome 8
• Robertsonian translocation


  Whole chromosome joined end-to-end with another, involving chromosomes 13, 14, 15, 21, and 22

  
• Gamete cells should only have one set of chromosomes to avoid doubling chromosome number during fertilization
• Organisms with an odd number of chromosome sets are usually sterile because their chromosomes cannot be divided evenly during meiosis

  
• Mechanism of polyploidy


  Failure of chromatid separation after metaphase

  Reduplication of chromosomes without nuclear membrane dissolution

  Failure of cytoplasmic division

  
• Mechanism of aneuploidy


  Non-dysjunction - Failure of chromosome separation during cell division

  Formation of 2 abnormal gamete types

  Fusion of abnormal gamete with normal gamete leads to trisomy or monosomy