p + q = Solved, Being the True Story of How the Chromosome Got Its Name

A few weeks ago I discussed different stories we have come to tell about the origins of the convention of using “p” and “q” to identify the chromosomal short and long arms, respectively (L’histoire de p et q: Urban Legends of Cytogenetics). That posting created quite a bit of discussion, particularly on the Cytogenetics Listserv. Several cytogenetics colleagues forwarded a comment from their listserv that should stand as the definitive story of how p & q became established as official cytogenetic nomenclature.

The True Version was related by someone who was in the room when the decision was made. In my previous post I alluded to the Battle of Hastings. That imagery was apparently not far off from the truth; one conference attendee reported that at times the session seemed like World War 2 1/2. As I had suspected, the 1966 Chicago Conference* was the scene of the crime. The suspects included some important figures in the history of medical genetics: Klaus Patau, Jérôme Lejeune, and Lionel Penrose.

Klaus Patau first described the clinical and cytogenetic basis of trisomy 13 in 1960, along with his wife Eeva Therman (the Finnish cytogeneticist who was sometimes called Mrs. X Chromosome), the great dysmorphologist Dave Smith, and two other authors. Patau originally worked at Germany’s Kaiser Wilhelm Institute and then went on to a long and illustrious career at the University of Wisconsin at Madison.

France’s Jérôme Jean Louis Marie Lejeune is known for his lifelong devotion to the study and care of people with Down syndrome. He was a devout Catholic and a friend and advisor to Pope John Paul II. In 1959, Lejeune, along with Raymond Turpin and Marthe Gautier, were the first to report the underlying cytogenetics of Down syndrome. Gautier, a physician who worked in Turpin’s laboratory at the Centre National de la Recherche Scientifique in Paris, first thought of the idea to karyotype a patient with Down syndrome patient and noted the presence of an extra chromosome in the cells cultured from a skin biopsy.  Lejeune identified the culprit as the twenty-first chromosome. Decades before, several scientists had suggested that Down syndrome might have a cytogenetic basis, including Petrus Waardenburg, Guido Fanconi, and Lionel Penrose, but the technology was not available at the time to test the theory.

Lionel Penrose, the longtime Galton Chair at the Galton Laboratory at University College London, was one of those great polymaths that England seems to be a breeding ground for. He was a psychiatrist, chess master, mathematician, medical geneticist, and, among other things, proposed a method (now called Penrose’s Law) for fairly allocating votes among countries in international organizations like the UN. He statistically established the association between advancing maternal age and an increased risk of Down syndrome.

Back in Chicago, the nomenclature session lasted many hours. Initially, “s” and “l” were recommended for the chromosomal short and long arms. Patau countered with “k” for kurz (German for “short”). Lejeune strongly argued for “p” for petite. This was followed by arguments about naming the long arm, with the concern that “l” could easily be confused for the number 1. In the wee hours of the morning, Penrose entered the room, wanting to know why the session had not yet ended. After hearing about the difficulties, he offered p & q because they were linguistically neutral, and because p + q = 1, evoking the idea that a short arm and a long arm together make one whole unit.

So it turns out that the True Story is closest to Version 4, what I labeled The Hardy-Weinberg version in my earlier post, but it also contains elements of Version 1 (The French Connection) and Version 2 (Francophones vs. Anglophones). Alas and alack, Version 3 (The NY Typesetter’s Error) appears to have no basis in truth, though it is still a good story. Interestingly, the results of the Voting Poll in my previous posting indicated that most people thought The French Connection was the correct story (garnering about 62% of the 211 votes cast by April 30th), whereas the Hardy-Weinberg story came in a distant third at about 11%, with The Francophones vs. The Anglophones attracting 19%, and the New York Typesetter’s Error coming in last with about 8% of the vote.

I guess that when it comes to history, there’s no such thing as The Truth, only distorted versions of it that over time become mistaken for the real thing. Of course, what is really interesting about history is not The Truth so much as the fact that we need to tell stories about our past, and those stories reflect complex personal, psychological, educational, and sociological factors.

* – Chicago Conference (1966): Standardization in Human Cytogenetics: Birth Defects. Original Article Series, Vol 2, No 2, New York, The National Foundation (1966).

Thank you to my colleagues from the Cytogenetics Listserv for forwarding the communication, and to the many readers of the DNA Exchange who took the time vote on The True Story and share their own stories.