1. Principles of meiosis and non-disjunction
Principles of meiosis and non-disjunction For simplicity, only one pair of newly replicated autosomes is shown in two different colours to distinguish the maternal from the paternal chromosome and crossover is not considered. During spermatogenesis, all four meiotic products can form the gametes (sperm), while in oogenesis, only one of the four products will actually become the ovum (egg) as one daughter cell forms a polar body at meiosis I (MI) and another forms a polar body at meiosis II (MII). For clarity, all four potential meiotic products are shown. (A) During normal meiosis, four haploid meiotic products are formed. (B) If non-disjunction occurs during MI, two daughter cells are formed which completely lack this particular chromosome (nullisomic for this chromosome), while two others contain two copies of the chromosome (disomic). (C) If non-disjunction occurs during MII, one nullisomic and one disomic daughter cell is formed, while the remaining two form normally.
Maria Jackson et al. Essays Biochem. 2018;62:
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