Mutations in the number of chromosomes can arise spontaneously by chromosome non-disjunction during meiosis
Sometimes chromosomes fail to separate properly during meiosis. This error, called non-disjunction, produces sex cells with the wrong number of chromosomes.
Real World
Down syndrome (trisomy 21) results from non-disjunction producing an egg with two copies of chromosome 21; when fertilised by a normal sperm, the resulting zygote has 47 chromosomes instead of 46.
Exam Focus
Specify at which stage of meiosis (I or II) non-disjunction occurred and show how gamete chromosome number leads to the trisomy in the zygote.
Evaluation Scaffold
A four-step framework for high-quality evaluation. Use this for 'assess', 'evaluate', and 'to what extent' questions.
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