Chromosome structure and function in man. 3. Pachytene analysis and indentification of the supernumerary chromosome in a case of Down's syndrome (mongolism).

Recently developed pachytene maps of the two small acrocentric autosomes (numbers 21 and 22) of man have been applied to a case of Down's syndrome mosaic for normal and trisomic cells (46,XY/47,XY,21+). Trivalents in trisomic spermatocytes, and thus the supernumerary chromosome, were recognized as compatible in length and chromomere pattern with the shorter of these two chromosomes at the pachytene stage. With the exception of the region of the centromere and the short arm, association among constituents of the trivalent appeared complete.