December 22, 1955

Joe Hin Tjio

Traditional date of Joe Hin Tjio’s 46-chromosome observation at Lund

A number accepted for three decades collapsed when clearer slides showed 46 chromosomes, not 48. The famous 2 a.m. timestamp is where historians now disagree.

The night attributed to 22 December

In Joe Hin Tjio's later account, he prepared unusually clear spreads of cultured human embryonic lung cells at about 2 a.m. on 22 or 23 December 1955, and counted 46 chromosomes rather than the accepted 48. He recalled summoning a Lund colleague, Carl Larsson, to inspect the slides.

The work took place in Albert Levan's Cancer Chromosome Laboratory at Lund University's Institute of Genetics, where Tjio was a visiting researcher from Zaragoza. Improved tissue culture, colchicine arrest, hypotonic treatment and squash preparation had produced spreads clear enough to count without the overlaps that had propped up the figure of 48.

What the logbook says instead

The exact 22 December attribution is a traditional date drawn chiefly from Tjio's retrospective account and dated photograph dedications, not from a contemporary laboratory entry. The Lund logbook records his return from Spain on 19 December.

Ulfur Arnason's 2006 reassessment argues that Levan made preliminary 46-chromosome counts around 20–23 December and that Tjio's conclusive preparations came in early January 1956. Peter Harper's historical review dates the determination broadly to December 1955, so the month is better supported than the day or the sole attribution.

261 counts, not one photogenic cell

Nothing changed publicly that night. Tjio and Levan repeated the counts and reported that 46 predominated in cultures from all four embryos studied — a result resting on 22 cell cultures and 261 exact chromosome counts.

Their short paper carries the dateline “Institute of Genetics, Lund, January 26, 1956,” which is not its publication date; Harper gives April 1956 and Wiley's bibliographic metadata gives May.

The authors wrote that they were surprised 46 predominated in every embryo's cultures.

Sources

Researched 23 Aug 2026 7 sources 2 audit passes Date disputed

How this was checked

Researched from the web into a fact sheet, rewritten from that sheet, then audited against it by a different model. How the pipeline works →

Date disputed. Sources disagree; the research places it at ~1955-12.

What the sources leave uncertain

December 22 is a traditional date derived chiefly from Tjio’s retrospective account and dated photograph dedications, not a contemporary laboratory entry.
The Lund logbook records Tjio’s return from Spain on December 19. Ulfur Arnason’s 2006 reassessment argues that Levan made preliminary counts around December 20–23 and Tjio made his conclusive preparations in early January.
Peter Harper’s historical review broadly dates the determination to December 1955, so the month is better supported than the precise day or sole attribution.
The paper ends with the dateline “Institute of Genetics, Lund, January 26, 1956,” but that was not its publication date. Harper gives April, while Wiley’s bibliographic metadata gives May 1956.