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  • Sometimes, scientists realize they are doing revolutionary work, and the world agrees.

  • Darwin and Pasteur, for example, were massive celebrities.

  • Other times, revolutionaries toil quietly for decades, leaving behind work that the

  • rest of us appreciate only much later.

  • This is the story of Gregor Mendel and the birth, loss, and rebirth of classical genetics.

  • [Intro Music Plays]

  • According to Darwin, organisms have slightly different traits, and this slight variation

  • becomes more important over time, as environments change and some traits become more useful

  • than others.

  • Organisms give traits to their descendants.

  • Over millions of years, new species split off as they become so different from their

  • ancestral species that they can no longer interbreed.

  • Sounds good!

  • But wait, how are traits passed down?

  • If a tall person marries a short person and they have a kid, how likely is that kid to

  • be tall, medium, short?

  • Darwin knew perfectly well that he didn't know.

  • He theorized the general category of thing that he thought he shouldor someone shouldfigure

  • out.

  • He called the hypothetical unit of heredity thepangene.”

  • This is where we getgene.”

  • But Darwin didn't know what a “geneshould look like.

  • Would there be a gene fortallorshort?”

  • Or a bunch of genes that somehow interacted to influence height?

  • Or was height all a product of what you ate as a kid?

  • Today, geneticists can answer these questions, in part thanks to a contemporary of Darwin's

  • who went largely unknown in his day.

  • Gregor Mendel was born in the Austrian Empire, in what is now the Czech Republic, in 1822—the

  • same year as Galton and Pasteur.

  • Mendel's family were poor farmers.

  • He was always interested in growing plants and beekeeping.

  • He went off to college to study philosophy and physics at Palacký University.

  • There, he studied with an agricultural scientist named Johann Karl Nestler who specialized

  • in breeding sheep.

  • But he ultimately became a monk at St. Thomas's Abbey.

  • Still, that didn't stop Mendel from studying science.

  • He asked his abbot for some land to set up an experimental garden, specifically to study

  • natural variation in English peas.

  • And from 1856 to 1863, that's what Mendel did.

  • ThoughtBubble, show us the wonders of counting English peas:

  • Mendel grew and tracked 28,000 plants.

  • He focused on seven traits: seed color, individual seed shape, unripe seed pod color, seed pod

  • shape, flower color, flower location, and plant height.

  • Importantly, these traits seemed to be inherited independently of each other, which made these

  • seven traits really useful for doing quantitative, or measurement-based, biology.

  • This work on peas wasn't that different from Darwin's pigeon breeding: both scientists

  • wanted to see how traits vary over time.

  • But you can grow more peas, faster, than you can pigeons.

  • So, after seven years of carefully tending peas, what did my dude conclude?

  • Mendel noticed that some characteristics seemed to be passed down often, and some tended to

  • disappear after only one generation.

  • He coined the termsdominantandrecessiveto describe these traits.

  • Putting numbers to his experiments, Mendel saw that 1 in 4 pea plants had purebred recessive

  • traits.

  • 2 in 4 were hybrids with both recessive and dominant traits.

  • And 1 in 4 were purebred dominant for the traits.

  • You can draw this as a square to help visualize thecrossesof the dominant and recessive

  • traits.

  • Mendel also figured out three general claims that are now known as the Laws of Menmdelian

  • Inheritance.

  • The first is the Law of Segregation, which states that the genes that control traits

  • are distinct.

  • Some of them, anyways.

  • The second is the Law of Independent Assortment: genes that control different traits switch

  • around when organisms breed.

  • Changing a pea's seed color in breeding, say, doesn't seem to change its height.

  • And the third Mendelian law is that of Dominance: some traits are dominant, and others recessive.

  • Thanks Thoughtbubble.

  • Mendel shared his pea results in a paper calledExperiments on Plant Hybridizationin

  • 1865.

  • And Mendel corresponded with the influential Swiss botanist Carlgeli from 1866 to 1873.

  • Boom!

  • Within one decade of Darwin's Origin, Wallace's Malay Archipelago, Galton's Hereditary Genius,

  • and Pasteur's experiments on biogenesisMendel had created a quantitative genetics.

  • And yetnobody cared.

  • Why the eclipse of poor Gregor?

  • First of all, Mendel himself didn't care, in the big-picture sense.

  • His goal had been to improve plant breeding.

  • In no way was he trying to be like Chuck Darwin and promote a grand theory of Life.

  • Second, Mendel was so isolated in a backwater abbey in eastern Europe, far from London or

  • Paris.

  • Third, Mendel just had super bad luck: he tried to reproduce the results of his pea

  • experimentsbecause, you know, the scientific method.

  • But his second model plant was hawkweed.

  • No one knew at the time, but unlike humans and mice and flies and peas, hawkweed reproduces

  • asexually.

  • Two parents don't neatly cross traits when they make offspring.

  • So, no Mendelian recessive and dominant traits.

  • No square.

  • Fourth, right after his hawkweed debacle, Mendel got promoted to abbot in 1868.

  • This sidelined him with administrative duties.

  • Mendel didn't publish after that, and he wasn't part of a larger scientific debate

  • about heredity.

  • He was just too busy to write a book like Origin.

  • He had an abbey to run.

  • And fifth and finally, Mendel was scientifically so far ahead of his time that other biologists

  • didn't see how his work with peas related to the grand sweep of evolution.

  • It just wasn't obvious.

  • So Mendel died, and genetics was lost.

  • For a few decades.

  • Who rediscovered Mendel?

  • Who didn't!?

  • Right around 1900, four different researchers working on the heritability of traits independently

  • read Mendel's landmark paper and understood just how critical his pea experiments had

  • been.

  • They became champions ofMendelism,” or the science of heredity, which was soon

  • renamed genetics.

  • The rediscovery of Mendel's research led to the formulation of a specific research

  • plan by these geneticists.

  • In 1900, Dutch botanist Hugo de Vries rediscovered Mendel's isolation of traits.

  • De Vries was already a famous biologist for popularizing Darwin's termpangene

  • for the unit of heredity, and for coming up with the termmutation.”

  • De Vries rejected the gradual blending of characteristics that others argued for.

  • He thought traits could jump around, because he could observe changes in his evening primroses

  • after only one generation.

  • Also in 1900, German botanist Carl Correns rediscovered Mendel.

  • Correns had been a student of Mendel's famous colleague, Nägeli.

  • Alsoalso in 1900, Austrian agronomist Erich von Tschermak rediscovered Mendel and developed

  • disease-resistant hybrid crops.

  • And then in 1901, American economist William Jasper Spillman published his own independent

  • high-fiving of Mendel in a paper calledQuantitative Studies on the Transmission of Parental Characters

  • to Hybrid Offspring.”

  • Which pretty much sums up classical genetics.

  • Just think about these events: one monk who loved gardening worked out how traits are

  • passed on in living things.

  • No one cared.

  • And then decades later, in the span of a single year, four separate researchers realized that

  • this monk's data on peas was absolutely priceless.

  • Retroactively, Mendel became thefatherof genetics.

  • Historians of biology have debated exactly how Mendel well really fits that title.

  • But, overall, his legacy was secured by de Vries and his contemporaries.

  • The work of the first geneticists also gave rise to a controversy in the life sciences.

  • On the one hand, those scientists who followed Darwin and Galton believed that traits blended

  • smoothly.

  • This is what Galton saw in human populations.

  • On the other hand, the geneticists like de Vries had extensive hands-on experience with

  • plant breeding and could see that Mendel was right: many traits jump around from generation

  • to generation.

  • But the botanists didn't make Mendel a famous science hero: the Fly Boys did.

  • In the 1910s, a group at Columbia University in New York led by Thomas Hunt Morgan conducted

  • extensive experiments on the genetics of fruit flies.

  • The scientists at Columbia's Fly Room researched mutations in the common fruit fly, Drosophila

  • melanogaster.

  • One of Morgan's star student's, AlfredHot DogSturtevant, pioneered genetic

  • linkage maps, or ways of finding the locations of genes on chromosomes, the tubelike physical

  • structures that store genetic material.

  • This involved painstakingly breeding flies with two different mutations and comparing

  • their chromosomes.

  • Linkage maps are markers of orderof which genes come after whichnot exact locations.

  • But they were still very useful in working out how traits are passed down.

  • With many, many, gross experiments going on, the Fly Room researchers needed a lot of flies.

  • They also had to develop standardized breeding practices.

  • Over many fly generations, theyreconstructedtheir flies into a standard type that could

  • be crossed with stable mutants.

  • This became the first real model organism, a living laboratory technology that could

  • be shared with distant colleagues, upgraded to surpass rivals, customized on demand, and

  • re-made easily in case of emergency.

  • Today, we have many other model organisms, including worms, mice, rats, rabbits, pigs,

  • monkeys, and everyone's favorite, bread mold.

  • Three of the Fly Guys authored The Mechanism of Mendelian Heredity in 1915, which became

  • the foundational textbook of classical genetics.

  • And Morgan won the Nobel Prize in Physiology or Medicine in 1933 for his lab's work on

  • the role that chromosomes play in heredity.

  • But the Nobelist who did the most work on how chromosomes transmit genetic informationin

  • an organism with way more chromosomes than fruit flieswas American geneticist Barbara

  • McClintock.

  • In the 1920s, she discovered how genes combineand thus how information is exchanged when cells

  • divide.

  • She produced the first genetic map for corn or maize, linking regions of the chromosome