Down's Syndrome

• What is Down's Syndrome caused by?

  A person having an extra copy of chromosome 21 (or an extra copy of part of chromosome 21)
• What does non-disjunction mean?

  Chromosome 21 fails to separate properly during meiosis, so one cell gets an extra copy of 21 and another gets none
• What happens when the gamete with the extra copy fuses to another gamete at fertilisation?

  The resulting zygote will have three copies of chromosome 21