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Explain the Mechanism and Consequences of Chromosomal Nondisjunction
In human genetics, nondisjunction is a critical error in cell division. Answer the following multi-part question thoroughly:
1. Define nondisjunction and explain precisely how it differs when it occurs during meiosis I versus meiosis II. Include a description of which specific cellular event fails in each case.
2. For a cell undergoing normal meiosis of a single chromosome pair (2n = 2), diagram in words the expected chromosome content of all four resulting gametes if nondisjunction occurs in meiosis I, and separately if it occurs in meiosis II. State the ploidy of each resulting gamete.
3. Explain why maternal meiosis I nondisjunction is more common than meiosis II nondisjunction for most human trisomies, referencing the role of the prolonged dictyate arrest in oocytes.
4. Trisomy 21 (Down syndrome), Trisomy 18 (Edwards syndrome), and Trisomy 13 (Patau syndrome) are the three autosomal trisomies compatible with live birth. Explain why trisomy of most other autosomes is lethal, invoking the concept of gene dosage imbalance, and explain why trisomy of smaller, gene-poor chromosomes is comparatively more survivable.
5. Distinguish between full trisomy, mosaic trisomy, and Robertsonian translocation trisomy using Trisomy 21 as your example. Explain how each arises and how their phenotypic severity may differ.
