Abstract

Association modalities of the three 21 chromosomes were studied during pachytene in three trisomy 21 fetuses whose chromosomal constitution was identified following amniocentesis. -- Three classes of images were observed: a trivalent, a trivalent presenting an important asynaptic region of the long arm, and a bivalent accompanied by a univalent. Such behaviour is analagous to that observed in all trisomic organisms. -- We have been able to establish the sequence of chromomeres, whose number varies from 9 to 14 according to the state of contraction in the 21 chromosome. Each band is thus subdivided into several sub-bands: at maximal elongation 2 sub-bands for band p11, 4 for q21 and 3 for q222. In addition, the interchromomeric clear bands q221 and q223 are also subdivided by the presence of a very small chromomere. In this way, the G-bands visible on mitotic metaphase chromosomes result from the compression together of several chromomeres whose individuality disappears as chromosomal condensation increases with progression of prophase.