BRAINCAKENon-disjunction
Non-disjunction is the failure of chromosomes to separate correctly during meiosis. It produces gametes with an abnormal number of chromosomes: one too many (n + 1) or one too few (n − 1).
It can happen in two ways:
• In meiosis I, a pair of homologous chromosomes fails to separate in anaphase I, and both move to the same pole. Of the four gametes produced, two have an extra chromosome (n + 1) and two are missing that chromosome (n − 1), so all four are abnormal.
• In meiosis II, the sister chromatids of one chromosome fail to separate in anaphase II. Of the four gametes, one has an extra chromosome, one is missing it and two are normal.
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Key terms in this lesson
- zygote
- The diploid cell formed by the fusion of a male and a female gamete at fertilisation.
More in Meiosis and Sexual Reproduction
- Telophase 1
- Meiosis 2
- The Importance of Meiosis
- Chromosome Mutation
- Down's Syndrome
- Turner's Syndrome
- Spermatogenesis
- Oogenesis
All 28 lessons in Meiosis and Sexual Reproduction · All Edexcel AS-level Biology topics