Final answer:
Aneuploidy can occur if nondisjunction occurs during both Meiosis I and Meiosis II, with nondisjunction in Meiosis I affecting all gametes and in Meiosis II possibly resulting in half normal and half aneuploid gametes.
Step-by-step explanation:
Aneuploidy may occur in some cells of an individual if nondisjunction happens during both Meiosis I and Meiosis II. Nondisjunction is the failure of homologous chromosomes or sister chromatids to segregate properly during cell division.
In the case of Meiosis I, nondisjunction occurs when a homologous pair of chromosomes does not separate, leading to gametes with an extra chromosome (n+1) or a missing chromosome (n-1). This results in all resultant gametes being aneuploid. Conversely, nondisjunction during Meiosis II occurs when sister chromatids fail to separate, which could result in two normal gametes and two aneuploid gametes – one with an extra and one with a missing chromosome.
A direct answer to the student's question is that aneuploidy can occur if nondisjunction happens during both Meiosis I and Meiosis II. It's also relevant to mention that conditions such as Down Syndrome are a result of aneuploidy, specifically trisomy 21, where there is an extra copy of chromosome 21.