Final answer:
Non-disjunction during meiosis leads to the production of gametes with an abnormal number of chromosomes, either extra or missing, resulting in aneuploidy when such gametes fuse during fertilization, as seen in conditions like Trisomy 21.
Step-by-step explanation:
How Non-Disjunction Leads to Aneuploidy
Non-disjunction is a process during meiosis where homologous chromosomes or sister chromatids fail to separate properly. This malfunction can occur in either meiosis I or II. During meiosis I, if non-disjunction occurs, a pair of homologous chromosomes does not separate, resulting in gametes that have one extra chromosome (n+1) or one less (n-1), leading to aneuploidy upon fertilization. Similarly, if non-disjunction occurs during meiosis II, it involves sister chromatids not separating, which can also result in gametes with an abnormal number of chromosomes. Thus, aneuploidy occurs when the resulting gametes fuse during fertilization, potentially leading to a zygote with 2n+1 (trisomy) or 2n-1 (monosomy) chromosomes.
Aneuploidy can have severe consequences on development and health, as seen in conditions like Trisomy 21 (Down syndrome), where an individual has an extra copy of chromosome 21.