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IB Biology SL D2.1.10 Down Syndrome Questions

Non-disjunction occurs when homologous chromosomes or sister chromatids fail to separate during meiosis. When chromosome 21 is affected, an extra copy can lead to Down syndrome (trisomy 21).

Syllabus
First assessment 2025
Course
Biology SL
Level
SL

Exam points

  • Explain how homologues failing to separate in meiosis I, or sister chromatids in meiosis II, produces gametes with abnormal chromosome numbers.
  • Relate non-disjunction of chromosome 21 to trisomy 21 and Down syndrome after fertilization.
  • Explain the link between maternal age and the increased risk of chromosome-21 non-disjunction.

IB Biology SL D2.1.10 Down Syndrome Questions question 1

[Maximum number: 2]

The following sequence of pictures, made using an electronic imaging technique, shows a cell undergoing division.

Image I

Image I

Explain, using one example, how non-disjunction in meiosis can lead to changes in chromosome number.

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