Daniel Hartl

Dr. Daniel L. Hartl is the Higgins Professor of Biology in the Departments of Organismic and Evolutionary Biology, Professor of Immunology and Infectious Diseases at Harvard University in the USA, and recipient of the Thomas Hunt Morgan Medal from the Genetics Society of America for lifetime achievement in genetics.  His research focuses on the interface between evolutionary biology and genomics, and he has also conducted considerable research on Mendel’s history and the foundational nature of Mendel’s experiments.  He is the author or co-author of numerous articles and textbooks on genetics and biology.

Painting by D.J. Fairbanks

You had the privilege of working closely and collaboratively with Vítězslav Orel, who was one of Mendel’s principal biographers, head of the Mendelianum, and a legendary researcher of Mendel’s history.  Could you share with us a few of your reminiscences of working with Dr. Orel, and the impact he had on solidifying Mendel’s legacy?

I first met Vítĕzslav Orel on Monday 21 May 1990.  I was at a meeting in Vienna and decided on a day trip to Brno to see the Abbey of St. Thomas where Mendel carried out his experiments.  Communist rule in what was then Czechoslovakia had ended a year earlier with the election of Václav Havel as president, and Americans were allowed to visit without a visa.  Accompanied by a generous young man who befriended me at the train station, we made our way to the abbey, and I wandered around to the western part of the refectory to see Mendel’s garden.  The door to the Mendelianum adjacent to the garden was open.  I entered and was greeted by a slightly built, grey-haired gentleman who introduced himself as Vítĕzslav Orel.  On learning that I was a geneticist, he guided me through the exhibits, showed me Mendel’s private quarters and books, and presented me with a memorial Medal from the 1965 Mendel Symposium that he had organised.  Afterward we adjourned to a local pub and enjoyed several cold Pilsners while discussing Mendel and what we thought were misinterpretations of Mendel’s work and his motivations then circulating among a few historians of science.  We decided to write a rebuttal based on a fresh analysis of Mendel’s paper.  At the time I was at Washington University School of Medicine in St. Louis.  Vítĕzslav visited exactly one year later, and we started writing the rebuttal in earnest 1.

Although Vítĕzslav’s mobility at our meetings in 1990 and 1991 seemed unimpaired, he had suffered a spinal-cord injury in 1989 in an automobile accident that resulted in a slowly progressing paraplegia that by 1994 had left him bedridden.  His wife Olga was a professional nurse who cared for him, and his son Marek installed an internet connection near his bed.  As his legs failed, “he was able to convey his suffering into work” 2.  He wrote a thorough analysis of the many controversies surrounding Mendel’s work 3 as well as his magnum opus, a biography of Mendel 4.  The latter demonstrates his erudition: he had used his years in the Mendelianum to become the world’s foremost authority on the history of selective breeding in Moravia, the prominence of the Abbey of St. Thomas as a scientific and cultural centre in Eastern Europe, and the background of Mendel’s work including that of Mendel’s mentor and superior (and one of Orel’s heroes), Abbot Cyrill František Napp.

You have a long career as an extraordinary teacher and textbook author.  How do you perceive Mendel’s experiments as an exemplary model for teaching not only genetics but also the nature of science?

My earliest reading of Mendel’s paper was as a first-year graduate student of James F. Crow at the University of Wisconsin, Madison.  The translation was that of 5.  Although it has since been superseded 6, it was the best available at the time.  I was taken then (and still am) by Mendel’s apparent honesty and forthrightness: He states clearly and concisely what he set out to do, why he set out do it, what he did, what he found, and what he thought it meant about heredity.  What a marvel of scientific communication!  What a pity that modern papers have been reduced to snippets of information for an audience of readers with ever-decreasing attention spans.

A few years later I had the privilege of getting to know Stern and Sherwood personally.  In 1969 I was awarded a postdoctoral fellowship at the University of California, Berkeley to work with Spencer W. Brown.  I soon became a regular member at the weekly group meetings of Professor Stern.  These were among the happiest days of my professional life.  Curt Stern was a small, balding, soft-spoken man with a piercing gaze whose influence in genetics was huge: he had suggested to Jaroslav Kříženecký and Vítĕzslav Orel that they organise the 1965 Mendel Symposium in Brno, now regarded as the spike in the heart of Lysenkoism.  His chief laboratory assistant was Eva Sherwood.  She was physically elegant and intellectually sharp.  Tall, slim, blond, and deeply tanned, Eva was exceptionally well organised and used color-coded cotton plugs as stoppers in Drosophila vials to indicate which experiments they belonged to and when they needed to be refreshed.

Mendel’s research revolutionised the study of heredity in two ways.  One was methodological: He demonstrated that the proper way to study heredity is character by character and trait by trait, paying careful attention to genealogy and the numerical ratios of contrasting traits in each generation.  The other was experimental: What he discovered is perhaps best summarised in his own words — “…pea hybrids form germ and pollen cells that, according to their nature, correspond in equal numbers to all the constant forms that arise from the combination of characters united through fertilisation6.  This neatly summarises both the laws of segregation and independent assortment in a form that is generally applicable to hybrids of any number of unlinked genes.

Mendel’s paper has been my lifelong model of how to do science as well as how to teach it.  I’ve never been quite sure whether I chose genetics as a profession or whether genetics chose me.  Genetics combines chemistry, mathematics, statistics, and biology in a way that I find deeply satisfying. I’m also attracted by how genetics highlights the role of chance in shaping our lives.  I don’t mean only the randomness of Mendelian segregation and recombination, but also the chance environments that influence how hereditary traits are expressed, especially complex traits.  I’d also include accidents of time and place that can change our lives.

Students learning genetics need to understand that genetics has its own episteme: its own system of advancing knowledge; its own way of asking questions, gathering evidence, and generating answers (e.g., mutant screens, genome-wide association studies).  In major new discoveries, more often than not, the original interpretations were wrong, but the question and evidence gathering was a major advance in itself.  The initially wrong conclusions were corrected by later investigators – which sometimes took years or even decades.  The lesson for us today is to be sceptical: Much of what we think we know will be altered or amended as time goes on.

My main objective in teaching genetics is for students to learn the basics.  As famous cookbook author Dorie Greenspan 7 has written: “When I was starting out, teaching myself to cook and bake and trying to make a career in food, I would corner every chef who would give me a minute and ask what I had to do to get good.  From line cooks at neighbourhood joints to chefs who wore starched whites, their advice was always the same: Learn the basics”.  To me, learning the basics of genetics means solving problems, designing experiments, completing individual activities, and group learning.


  1. Hartl DL, Orel V (1992) What did Gregor Mendel think he discovered? Genetics 131:245–253
  2. Paleček P (2016) Vítĕzslav Orel (1926–2015): Gregor Mendel’s biographer and the rehabilitation of genetics in the Communist Bloc. Hist Phil Life Sci 38:4.
  3. Orel V, Hartl DL (1994) Controversies in the interpretation of Mendel’s discovery. Hist Phil Life Sci 16:423–464
  4. Orel V (1996) Gregor Mendel: the first geneticist. Oxford University Press, Oxford.
  5. Stern C, Sherwood ER, Eds (1966) The origin of genetics: a Mendel source book. W. H. Freeman, San Francisco
  6. Abbott S, Fairbanks DJ (2016) Experiments on plant hybrids by Gregor Mendel. Genetics 204:407–422.
  7. Greenspan D (2018) https://www.nytimes.com/2018/08/08/magazine/french-pastry-techniques-custard.html (accessed 9 June 2022).