how can non-disjunction lead to the production of an egg cell with an extra chromosome
Non-disjunction is when homologous chromosomes fail to separate during meosis I or II. The result is that the unseparated chromosome pair will enter one daughter cell, while the other daughter cell will not get that chromosome at all. For example, if this were a human gamete, there would be either 22 or 24 chromosomes in the abnormal gamete. This condition is called aneuploidy.