Mutations in Chromosome Number (AQA A Level Biology): Revision Note

Mutations in chromosome number

• Mutations can occur at different levels, within chromosomes or at the genetic level

• Chromosome mutations involve a change in the number of chromosomes and involve non-disjunction

• 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 fertilisation, then a chromosome mutation occurs, as the resulting diploid cell will have the incorrect number of chromosomes

• An example of a chromosome mutation is Down’s syndrome

  • Individuals with this syndrome have a total of 47 chromosomes (instead of 46) in their genome, as they have three copies of chromosome 21