[Oocyte meiosis in a trisomy 18 fetus. Behavior of the supernumerary chromosome and identification of the 18 bivalent]

Associations of the three n degrees 18 chromosomes were studied in a trisomy 18 fetus (the chromosomal constitution of which had been identified by amniocentesis). The three classes of associations observed were those observed in other trisomic organisms:trivalent, trivalent presenting an important asynaptic region, and bivalent accompanied by a univalent. In addition, the sequence was established of chromomeres, the number of which varied from 18 to 23 depending on the degree of chromosome contraction. In elongated pachytene oocyte bivalents each G-band of mitotic metaphase chromosomes could be subdivided into several sub-bands.