Genes, Chromosomes and Cell Division

Chromosome abnormalities

Balanced chromosome rearrangement

  • All the chromosomal material is present

Unbalanced chromosome rearrangement

  • Extra or missing chromosomal material, usually 1 or 3 copies of gene
  • Causes major developmental problems

Aneuploidy

  • Whole extra or missing chromosome
  • X chromosome aneuploidy better tolerated because of X chromosome inactivation

Translocation

  • Rearrangement of chromosomes
Robertsonian translocation
  • Two acrocentric chromosomes stuck end to end
  • Increased risk of trisomy in pregnancy
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Reciprocal translocation
  • Two broken off chromosome pieces of non-homologous chromosomes are exchanged
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Specific karyotypes

  • 47XY +21 (trisomy 21) - Down’s syndrome
  • 47 XY +14 (trisomy 14) - miscarriage
  • 47 XY +18 - Edward syndrome, trisomy 18
  • 45 X - Turner syndrome
  • 47 XXY - Klinefelter syndrome

Insertions and deletions

  • Missing or duplicated material
  • Microdeletions - too small to be seen on chromosome using light microscope

Somatic mosaicism

  • Refers to the occurrence of two genetically distinct populations of cells within an individual, derived from a post-zygotic mutation
  • May only affect a portion of the body
  • Not transmitted to progeny
  • Somatic mosaicism for a chromosomal abnormality could contribute to cancer
    • Changes could activate an oncogene or delete a tumour suppressor