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.
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.