Down syndrome causes

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Causes

Down's syndrome is caused by triplication of chromosome 21 (trisomy 21). The aneuploidy may occur in three possible ways:

• Meiotic nondisjunction of homologous chromosomes
  • From improper division of chromosomes during meiosis I or meiosis II. One gamete will end up with extra 21st chromosome.
• Robertsonian translocation
  • Individuals with normal number of chromosomes but an extra portion of 21st chromosome.
• Mosaicism
  • Post-fertilization mitotic error