The Sites and Relative Frequencies of Secondary Constrictions in Human Somatic Chromosomes

Abstract

Karyotype analyses made from photomicrographs of 1303 human somatic cells derived mainly from peripheral leucocyte cultures obtained during the investigation of patients for chromosome aberrations, have been re-examined in order to determine the sites and relative frequencies of secondary constrictions. It has been confirmed that the chromosomes showing secondary constrictions most frequently in this material are chromosomes 13, 14, 15, 21 and 22. In these five pairs, a secondary constriction in the short arm separates a terminal mass, the satellite, from the rest of the chromosome arm. Secondary constrictions were also observed at specific sites in other chromosomes, although less frequently. In chromosomes 1, 6, 9 and 17 the constriction peculiar to each appeared in over 20% of the available chromosomes of the respective chromosome pair. Examples of secondary constrictions in these and other chromosomes are illustrated. Our observations provide evidence of at least fifteen sites, where constrictions are visible at a definite point with sufficient frequency to exclude any explanation other than that they are a distinct entity, characteristic for each chromosome. For each of these 15 constrictions, 10 chromosomes have been measured and the distance from the centromere to the constriction determined. The characteristic site of each constriction is indicated in a “normal human idiogram” constructed from chromosome measurements in 10 selected male cells. The value of secondary constrictions in the identification of human chromosomes is discussed, and the importance of recognising their location and variable appearance is stressed in view of the possibility that they may on occasions be interpreted as evidence of chromosome aberration.