D2.1.10

• If the chromosomes all separate correctly, four haploid cells are produced by meiosis in a diploid cell, however errors may occur.
• Non-disjunction occurs when either homologous pairs of chromosomes fail to separate during meiosis I or when sister chromatids fail to separate during meiosis II.

  
• Usually a missing chromosome quickly leads to the death of a haploid cell, and having an extra chromosome is also a lethal condition in most cases.
• Sometimes an extra chromosome is not lethal, for example: Down syndrome, where there is a third copy of the 21st chromosome. Other examples may be Klinefelter’s syndrome or Turner’s syndrome.