AQA AS Biology

Revision Notes

4.3.2 Mutations in Chromosome Number

Mutations in Chromosome Number

  • Mutations can occur at different levels
  • Non-disjunction occurs when chromosomes fail to separate during meiosis
    • This occurs spontaneously
  • The gametes may end up with one extra copy of a particular chromosome or no copies of a particular chromosome
  • These gametes will have a different number of chromosomes compared to the normal haploid number
  • If the abnormal gametes take part in fertilization, then a chromosome mutation occurs as the diploid cell will have the incorrect number of chromosomes
  • Chromosome mutations involve a change in the number of chromosomes
  • An example of chromosome mutation is Down’s syndrome: Individuals with this syndrome have a total of 47 chromosomes in their genome as they have three copies of chromosome 21

Nondisjunction, downloadable AS & A Level Biology revision notes

Image showing how chromosomes failing to separate properly during meiosis can result in gametes with the incorrect number of chromosomes

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.
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