Six decades ago, as the implications of the double helix were being pondered over, another basic discovery in medical genetics was quietly made.

Since the 1920s, the standard chromosome count of the human cell had been thought to be 48. Given the techniques available at the time, it’s surprising the count was as close as it was. A serendipitous discovery in the early 1950s – that pre-treatment of the specimen with a hypotonic solution markedly improved the separation and delineation of chromosomes – now allowed for accurate and reproducible recognition of their number, configuration, and relative size. Several investigators by 1956 reported consistent counts of 23 pairs or a diploid count of 46, including the sex chromosomes.

This finding by itself may not have been enough to excite much interest outside of the cytologists themselves. But the very reliability and stability of the count now allowed recognition of its occasional instability. In 1959, J. Lejeune reported trisomy of chromosome 21 in his Down Syndrome patients, and this clinical application created a sensation. Other chromosomal defects were soon found, and medical cytogenetics was off and running.

Submitted by Ron Fishman from the Cogan Ophthalmic History Society.