Process of Non-Disjunction Leading to Chromosomal Disorders

Key Points

• Non-disjunction at Meiosis I: entire homologue pair goes to one pole → aneuploid gametes
• Non-disjunction at Meiosis II: sister chromatids fail to separate → same result
• Maternal non-disjunction causes ~95% of Down syndrome cases
• Risk increases with maternal age (prolonged meiotic arrest in oocytes — arrested since fetal life)
• Turner syndrome can arise from maternal OR paternal non-disjunction