Chromosome mutations

• Chromosome mutations


  Changes in the number of chromosomes
• Nondisjunction in meiosis


  1. Chromosomes/chromatids do not separate equally in anaphase

  2. Results in gametes with incorrect chromosome number
• Polyploidy


  Whole additional set(s) of chromosomes
• Aneuploidy


  Change in number of just one chromosome
• Aneuploidy example


  • Down syndrome
• Polyploidy nondisjunction in meiosis I


  1. Chromosomes fail to separate equally

  2. One cell gets all chromosomes, other cells have no chromosomes

  3. Meiosis II proceeds normally, resulting in diploid gametes
• Polyploidy nondisjunction in meiosis II


  1. Chromatids fail to separate equally in one cell

  2. Results in diploid gamete
• Aneuploidy nondisjunction in meiosis I

  1. One chromosome pair fails to separate equally

  2. Results in gametes missing or having an extra copy of that chromosome
• Aneuploidy nondisjunction in meiosis II


  1. Chromatids of one chromosome fail to separate equally

  2. Results in gametes missing or having an extra copy of that chromosome
• Polyploidy is usually fatal in humans but common in plants
• Down syndrome is caused by trisomy (3 copies) of chromosome 21
• Aneuploidy can be caused by nondisjunction or non-disjunction, where homologous pairs fail to separate during anaphase.
• Nondisjunction occurs when sister chromatids do not separate properly at metaphase II (MII) of meiosis.
• In Down Syndrome, there are three copies of chromosome 21 instead of two.
• The most common cause of Down Syndrome is maternal nondisjunction during meiosis I, resulting in a fertilized egg with 69 chromosomes instead of 46.
• Non-disjunction during meiosis II results in gametes with either one or three copies of the affected chromosome, leading to offspring with Down Syndrome if they receive the abnormal gamete from their mother or father.