Trisomy is the single most frequent type of chromosome abnormality in humans and has considerable impact on many aspects of human pathology. It arises most commonly through “nondisjunction” at maternal meiosis I, but the underlying mechanism of formation remains obscure. Analysis of 100 haploid oocytes at second meiotic metaphase shows that the only type of chromosome abnormality compatible with trisomy formation after fertilisation is the presence of single chromatids in addition to, or replacing, whole chromosomes. The mechanism resulting in the presence of single chromatids is considered to be precocious division of univalents or dyads at first meiotic anaphase.