The Relative Length and Arm Ratio of the Human Late-replicating X Chromosome.

The human chromosome complement at metaphase of mitosis may be divided into 10-I2 sets. The chromosomes are conventionally assigned to the various groups according to the criteria of length and arm ratio. It has not yet proved possible, using these criteria, to separate the whole of the chromosome complement into 23 pairs of homologous chromosomes. The chromosomes of the X-6-I2 group are particularly difficult to pair. Recent developments in technique aid the identification of some of the chromosomes in the X-6-12 group. These techniques are the incorporation of 3H thymidine and the enhancement of secondary constrictions. It has been shown, in the normal female, that one chromosome in the X-6-I2 group incorporates 3H thymidine at a later stage of DNA synthesis than the other members of this group (Morishima, Grumbach, and Taylor, I962; Gilbert, Muldal, Lajtha, and Rowley, I962; German, I962). Evidence that this is an X chromosome is obtained also from studies on individuals with abnormal X chromosome complements (Gianelli, I963; Rowley, Muldal, Gilbert, Lajtha, Lindsten, Fraccaro and Kaijser, I963; Grumbach, Morishima, and Taylor, I963; Atkins, Book, Gustavson, Hansson, and Hjelm, I963). In this report we have studied the variation in length, arm ratio, and position within the X-6-12 group of the late replicating X chromosome (late X).