A Brief History of Gregor Mendel

I learned recently that Gregor Mendel – who was born 196 years ago Friday – wanted to be what I am: a teacher.

The poor guy tried. Took the qualifying exam twice, and failed both times. (He kept botching the section on “natural history.” Cue Alanis Morissette.) So he put in a few years as a substitute, and eventually left the job behind to focus on his true calling of gardening.

Well… that, and revolutionizing biology.

From 1856 to 1863, Mendel labored in a 120-by-20-foot patch of soil to undertake a prolonged study of pea plants. In thousands of painstaking experiments, he bred the plants together: dusting one’s pollen upon another’s stigma with a camel-hair pencil, handling his “children” (as he called them) with exquisite care.

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Mendel had identified seven easy-to-measure traits: the seeds’ shape, the color of their coating, the position of the flowers, and so on. By charting these traits across the generations, he sought to understand the patterns of inheritance. His findings, recorded by hand in two-penny notebooks, went something like this.

Begin with two pure-breed strains of plants: one with smooth, “round or roundish” seeds; the other with seeds that are “irregularly angular and deeply wrinkled.” When kept apart, each type produces more of its own: round yielding round, wrinkled yielding wrinkled. Generation after generation, nothing changes. Then along comes Mendel to breed them together.

The result? All of the offspring, down to the very last plant, exhibit round seeds.

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As Mendel notes, there are no “transitional forms” whatsoever. No resulting seeds are kinda-round-but-kinda-wrinkled. Instead, in every experiment Mendel ran, the progeny resembled “one of the parental forms so closely that the other either escapes observation completely or cannot be detected with certainty.” It’s as if the wrinkled trait has vanished entirely, dominated by the round form.

Weird enough. But it gets even weirder when Mendel breeds these round-seeded hybrids together. In the new generation, the wrinkled seeds reemerge—but in smaller proportion. The round seeds now outnumber them by a ratio of 3 to 1.

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It wasn’t just seed shape. Mendel found this 3-to-1 ratio cropping up in every experiment:

Trait First Generation Ratio in the
Second Generation
Seed shape:
Round vs. wrinkled
100% Round 2.96 to 1
Seed color:
Yellow vs. green
100% Yellow 3.01 to 1
Seed-coat color
White vs. gray-brown
100% Gray-brown 3.15 to 1
Pod shape:
Inflated vs. constricted
100% Inflated 2.94 to 1
Pod color:
Yellow vs. green
100% Green 2.82 to 1
Flower position:
Spread out vs. bunched
100% Spread out 3.14 to 1
Stem length:
Tall vs. short
100% Tall 2.84 to 1

The puzzle was profound. How could the secondary trait disappear, only to come back a generation later? The wrinkled seed, the constricted pod, the bunched-up flowers—these traits must somehow lie dormant in the first-generation seeds, a form of latent potential, information retained but not acted upon. Such results made no sense if a single factor determined each quality. This prompted Mendel’s first powerful insight.

Each trait must be determined by two factors.

What those “factors” were, Mendel didn’t yet know. (It would be decades before the word “gene” emerged.) Whatever they are, Mendel said, let’s give them names: “A” for the round-seed factor, and “a” for the wrinkled seed. Now, if each plant has duplicate factors, then the pure-breeds will be “AA” and “aa.” When bred together, the offspring receives one factor from each parent, for a combination of “Aa.”

But this does not result in a mingled, transitional state. The hybridity, the conflict between the two instructions, is all on the interior. Outside, roundness dominates, and the plant is visually indistinguishable from the round pure-breed.

What happens when two hybrids are bred together, “Aa” with “Aa”? Then, assuming the offspring receives one factor from each parent, four combinations are possible: “AA,” “Aa,” “aA,” and “aa.

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The first three will have round seeds, while the last will have wrinkled ones. That explains the characteristic 3-to-1 ratio that Mendel observed.

Perhaps this all strikes you as a big inferential leap, like claiming to know a poker player’s entire hand from their opening wager. Sure, the two-factor structure explains the observed data, but is that enough? Every good scientific theory makes testable predictions. What predictions does Mendel make, and do they prove true?

Here’s one: take a look at those round-seed offspring from the two hybrid parents. Although indistinguishable on the outside, Mendel’s theory implies that one-third of them are “AA” (restored to their original unmixed state) while two-thirds are “Aa” (hybrids like their parents). The former should have only round-seeded offspring, while the latter should yield progeny in the familiar 3-to-1 ratio. And indeed, when Mendel took his camel-hair pencil and his otherworldly patience to this question, this is exactly what he found.

I don’t know what it’s like to be a visionary scientist unjustly ignored in your own time, but it sounds lonely. I’d rather be a mediocre scientist unjustly celebrated in my time. Alas, nobody gave Gregor Mendel that choice: the founding father of genetics died in 1884 having never heard the word “genetics” (coined in 1905).

And that means he never witnessed, nor could have imagined, the sequence of discoveries soon to unfold:

  • 1889—Scientists first document the separation of chromosomes during cell division. These mysterious black structures appear just before a cell splits, line up down the center of the nucleus, and then migrate to opposite poles. Intriguing, but so far inexplicable.
  • 1903—A hypothesis emerges: Perhaps chromosomes carry the factors of inheritance that Mendel identified?
  • 1910—The hypothesis is verified. Genes reside on chromosomes.
  • 1913—The first chromosomal map is drawn. Genes appear to be located in a consistent linear sequence on a chromosome, like chapters in a book.
  • 1920s-1930s—Not much progress. Everyone too busy developing harebrained eugenicist theories and/or jazz chops.
  • 1941—It finally becomes clear what genes do: provide instructions for building proteins.
  • 1944—Genes are made of DNA.
  • 1953—DNA takes the shape of a gorgeous double helix.

This Mendel-spurred series of increments and revelations has delivered us into a bizarre sci-fi reality. Our species can now convert ancient mysteries (the nature of family, the patterns of inheritance, the connections between parent and child) into concrete scientific questions. We are like newly sentient robots, poking around in our programming. So much of the individuality, complexity, and richness of humanity can be traced back to our 48 chromosomes.

Oops—did I say 48? Sorry, we were just getting there:

  • 1955—An Indonesian biologist notices that the historically accepted number of human chromosomes (24 pairs, for a total of 48) had stemmed from a careless counting error. The actual number is 23 pairs, for a total of 46.

(If you’ve ever tried to take a roll call of 11-year-olds, it won’t surprise you that the hardest part of unlocking the secrets of the genome is getting an accurate head count.)

Gregor Mendel, deemed unqualified to teach a high school biology class, settled for conducting experiments now taught in every biology class in the world.

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10 thoughts on “A Brief History of Gregor Mendel

  1. 1900-01—Three biologists and an agronomist, independently of each other and of Mendel, repeat his experiments (in general, not with pea plants specifically), rediscover his laws of inheritance, and publish. All four eventually admit Mendel’s priority, with varying degrees of reluctance. Hugo de Vries, one of the biologists, is barely remembered for his early work on mutations; the other three are utterly forgotten.

  2. Oops, saved too soon.

    1866—Mendel publishes his work in the Proceedings of the Natural History Society of Brünn (now Brno in Czechia). He is totallly ignored.

    1. Ah, true. I still think he should have an extra “Science” category. Not just for this post, but for some others as well.

  3. Another interesting — and very mathematical! — thing about Mendel is the whole controversy about how the data he reported from his experiments is improbably good. When you say “The round seeds now outnumber them by a ratio of 3 to 1,” in fact there is some randomness and the ratio will not be *exactly* 3 to 1, in the genetic version of this biological situation … and yet Mendel’s data was *really* close to exactly 3 to 1. If you look carefully at all of his reported data, it starts to seem like he either had just amazingly good luck with how the dice rolled for his “children,” … or he somehow fudged his results… a little … maybe? It’s quite a conundrum, but relates interesting statistical analysis to real-world issues, so it seems ripe for a Math With Bad Drawings exposition!

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