During which stage of meiosis does aneuploidy primarily arise?

Aneuploidy primarily arises during maternal meiosis I due to the improper segregation of chromosomes. This process involves homologous chromosomes separating to different daughter cells. Errors in this segregation can lead to gametes with an abnormal number of chromosomes.

During maternal meiosis I, the separation of homologous chromosomes is a critical event. If the chromosomes fail to disjoin properly, known as nondisjunction, it results in one daughter cell receiving both copies of a chromosome and the other receiving none. This can lead to gametes that, when fertilized, result in an embryo with an abnormal number of chromosomes, characteristic of aneuploidy, such as Down syndrome, which is caused by an extra copy of chromosome 21.

While errors in paternal meiosis can also lead to aneuploidy, the predominant stage associated with this occurrence is maternal meiosis I. This is largely due to the fact that maternal meiosis has a longer duration and is more susceptible to age-related chromosomal segregation errors, compounding the risk of aneuploidy.