Hsu, 1952Hsu, 1956Denver
              System, 1960

Human karyotypes: 2n = 48 or 46?

    Early studies of the human karyotype simply stained chromosomes within cells with Giemsa and "squashed" them between the cover slip and slide. Most cells were not at the proper mitotic phase for chromosomes to be observed, and chromosome separation was poor. The exact count was uncertain: most workers accepted the number 48. The breakthrough came in 1952 (left) when a technician in T. C. Hsu's lab accidentally substituted distilled water for the normal saline solution used in washing the cells just before "squashing". This "hypotonic" treatment caused the cell nuclei to swell, and allowed the chromosomes to separate before squashing. A further refinement was "dropping" the cells onto the slide at arm's length, which caused the nuclei to burst on impact, further separating them (middle slide). Finally, the use of a plant spindle-poison Colchicine allow chromosomes to be arrested at mitotic metaphase, during their maximum state of compaction [right] These experiments quickly established the human chromosome number as 2n = 46 chromosomes.

    Classification of  chromosomes into seven groups by size and relative centromere position established the so-called "Denver System" (right) in 1960. Chromosomes within groups B - G are typically not distinguishable from each other. The X chromosome is in the C group, and the Y is in the G group. Modern banding techniques allow each chromosome in the karyotype to be distinguished individually.

Figure ©1979 by T. C. Hsu; all text material ©2011 by Steven M. Carr