4.3.2 Mutations in Chromosome Number

1. Polyploidy

• Definition: Polyploidy occurs when there is a change in the whole set of chromosomes, resulting in an organism having three or more sets of chromosomes instead of the usual two.
• Common in Plants:
  • Many modern plant species, such as wheat, are polyploidy, which often provides advantages like greater size and resistance to environmental factors.
• Rare in animals due to the complexity of balancing additional sets of chromosomes.

2. Non-Disjunction

• Definition: Non-disjunction occurs when homologous chromosomes (in meiosis I) or sister chromatids (in meiosis II) fail to separate properly during cell division.
• Effects:
  • This results in gametes (and zygotes formed from these gametes) having either one extra or one missing chromosome.
  • Example: Down's Syndrome, where individuals have an extra copy of chromosome 21 (trisomy 21).