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Nondisjunction — which phase is most dangerous?
I need to know: in which phase of meiosis does nondisjunction usually happen? Is it more common in meiosis I or II? And why does it cause more problems when it happens in mothers than fathers? Thank you!
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Nondisjunction means chromosomes fail to separate properly. It can happen in both divisions but statistics show differences.
Most common is nondisjunction in meiosis I — homologous chromosomes don’t separate → both homologues go to one daughter cell. This usually causes trisomy or monosomy of whole chromosomes (like Down syndrome — trisomy 21).
Nondisjunction in meiosis II is less common — sister chromatids fail to separate → one gamete gets two identical chromatids, another gets none. This can cause trisomy too but often of smaller chromosomes or mosaicism.
Why maternal age effect is stronger: in females meiosis I starts in fetal life and arrests for decades in prophase I. With age spindle apparatus and checkpoints get worse → more mistakes in meiosis I. In males meiosis happens continuously and new sperm are made all the time → less age-related errors.
Very clear diagrams showing where nondisjunction can occur and resulting karyotypes are here: phases of meiosis
Also maternal nondisjunction is responsible for about 90% of trisomy 21 cases. Paternal mostly affects sex chromosomes (Klinefelter, Turner). Hope this answers your question!