Chromosome anomalies in human oocytes in relation to age.

Cytogenetic analysis of oocytes remaining unfertilized after in-vitro fertilization showed that the source of data obtained could be divided into degenerating and 'healthy' oocytes. The degenerating oocytes, which showed different degrees of chromosome breakage, accounted for a quarter of the total. They were found in older patients with a mean age of 35.0 years. The healthy oocytes without chromosome breaks were mostly haploid and fell into two main groups, those with a normal MII,23,X chromosome complement, and those abnormal in which single chromatids replaced a whole chromosome. No oocytes hyperhaploid for an extra whole chromosome were found. We hypothesize that the single chromatids at second meiotic metaphase arise by precocious division of chromosome univalents at anaphase I (predivision) and that this may be the major mechanism for trisomy formation in man, rather than the non-disjunction of whole bivalents as generally assumed.