Since the 1 920s, when researchers began to study chromosomes, the analysis of human chromosomes has presented a particularly tough technological challenge, simply because there are so many of them. When displayed under a light microscope. the strands of human genetic material tend to bunch together maddeningly, overlapping and intertwining like so much spaghetti. For these reasons, it wasn’t until 1956 that the correct number of 46 human chromosomes (23 pairs) was clearly demonstrated. Since then, the field of cytogenetics—linking chromosome abnormalities with syndromes—has proceeded more or less in fits and starts, with key developments in chromosome preparation explaining more and more once mysterious medical conditions.

Key to connecting chromosomes to symptoms and traits is the karyotype, a size-order alignment of chromosome pairs in a chart. The first such efforts to align the pairs, however, were quite crude. By 1959, about all that could be discerned was an extra or...

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