Today, any medical textbook would correct this, but until the mid-20th century, the accepted scientific consensus was that humans had 48 chromosomes. The assumption persisted for more than three decades, despite doubts raised by the underlying specimens. The chromosome count was corrected in 1956. Joe Hin Tjio (1919-2001), cytogeneticist, and Albert Levan (1905-1998), Swedish botanist, geneticist, and cancer researcher, showed that normal human somatic cells have 46 chromosomes. Stanley M. Gartler, molecular biologist and human geneticist from the Departments of Medicine and Genome Sciences, University of Washington in Seattle, later reconstructed how the error arose.

Chromosomes had been visible since the second half of the 19th century, after fixation and staining techniques were introduced into cell biology. However, visibility does not make it easy to count. Outside cell division, chromosomes do not appear as clearly separated, compact structures. Even during mitosis, they often overlap in early preparations, whereas tissue sectioning can cut through cells and chromosome structures.

Therefore, it was unsurprising that estimates of human chromosome number varied widely. By 1914, at least 15 researchers had attempted to count human chromosomes and published their findings. Many arrived at 24. Hans de Winiwarter (1875-1949), Belgian gynecologist and cytologist, however, reported 47 chromosomes in testicular tissue and 48 in fetal ovaries. He appeared to have developed a method for rapidly fixing fresh tissue that limited chromatin clumping. These conflicting findings even fueled the speculation that individuals from different racial backgrounds might have different chromosome numbers.

Counting Difficulties and Textbook Number

Theophilus Shickel Painter (1889-1969), cytologist at The University of Texas at Austin, played a decisive role in establishing 48 as the accepted number. He worked mainly with human testicular tissue because it contained many dividing cells. Obtaining fresh, healthy human testicular tissue was a major obstacle for early human chromosome research.

When Painter published his first findings in 1921, accurately counting chromosomes using available preparation techniques was extremely difficult. The fixed tissue was embedded in paraffin, cut into thin sections, and stained. Cells could be cut across, and chromosomes were often tightly stacked on top of one another. Depending on the specimen, Painter counted between 45 and 48 chromosomes and ironically found only 46 chromosomes in the clearest images. Nevertheless, in 1923, he concluded that 48 was the correct human chromosome number.

Despite these uncertainties, Painter’s findings carried considerable weight. His research also dispelled an earlier theory that individuals of African and European ancestry might have different chromosome numbers: He found the same number in Black and White individuals. His finding of 48 chromosomes became the accepted standard and was included in textbooks for decades.

Why did the number 48 persist for so long? Gartler identified expectations as a key factor. Researchers who expected to observe 48 chromosomes could interpret difficult-to-read specimens accordingly. Despite conflicting findings and illustrations, the established number was not seriously questioned for decades.

Cytogenetic techniques continued to advance technologically. The “squash preparation” technique allowed chromosomes to be spread more effectively across a microscope slide than with the paraffin sectioning technique. An even more important factor was treatment with a hypotonic solution, which caused cells to swell and helped spread the nuclear content, separating individual chromosomes. Colchicine was also used to arrest dividing cells in metaphase, when chromosomes are the most condensed and therefore easier to analyze. These advances reduced chromosome overlaps and improved the reliability of chromosome counts.

Levan had already studied the effects of colchicine in plants. In the 1950s, he increasingly turned to cancer cytogenetics and helped improve the preparation techniques for human cells. Notably, even before his joint publication with Tjio, Levan referred in his own work to the “expected” normal chromosome number. Even advances in technology did not quickly dislodge this established expectation among researchers.

Born in Java, Indonesia, Tjio initially worked as an agronomist and plant cytogeneticist. After World War II, he continued his research in Europe and worked regularly with Levan on Lund, Sweden. Tjio also had experience with photography, a skill that later helped him document his chromosomal preparations.

In late 1955, Tjio examined fetal cell cultures in Lund, which were better suited to chromosome analysis than many of the tissue samples he had studied earlier. He was looking for reliable reference values for his cancer research and repeatedly counted 46 chromosomes.

Sam Kean, science author, described Tjio as a night owl who often worked from sunset until dawn. Drawing on his experience in his father’s darkroom, Tjio photographed microscopic images and enlarged them for closer examination.

It is not simply Tjio’s persistence in counting chromosomes that has proved decisive. Gartler emphasized the exceptional quality of the preparations, noting that the metaphase plates left little doubt that the cells contained 46 chromosomes. In January 1956, Tjio and Levan published their findings in Hereditas.

Modern Cytogenetics

The significance of the discovery lay primarily in the methodological advances it represented. Human chromosomes could now be visualized with sufficient reliability to distinguish 46 from 48 chromosomes with confidence. In 1956, Charles Ford and John Hamerton, cytogeneticists, confirmed this finding using testicular tissue, providing a methodological basis for reliably detecting numerical chromosome abnormalities.

Just a few years later, the clinical significance of cytogenetics became apparent. In 1959, characteristic chromosomal abnormalities were identified in Down, Turner, and Klinefelter syndromes. Shortly thereafter, chromosome analysis became important in cancer research. In 1960, Peter Nowell, American pathologist, and David Hungerford, cytogeneticist, described what would later be called the Philadelphia chromosome in chronic myeloid leukemia.

Later, chromosome banding techniques made it possible to reliably distinguish individual chromosomes and identify structural changes with much greater precision. In 1973, Janet Rowley, American geneticist, was therefore able to show that the Philadelphia chromosome was not the result of a simple deletion but rather of a translocation between chromosomes 9 and 22. Medical cytogenetics has evolved from a focus on determining the correct chromosome number to an established field of clinical research.

This story was translated from Univadis Spain, part of the Medscape Professional Network.