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 should—or someone should—figure

out.

He called the hypothetical unit of heredity the “pangene.”

This is where we get “gene.”

But Darwin didn't know what a “gene” should look like.

Would there be a gene for “tall” or “short?”

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 terms “dominant” and “recessive” to 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 the “crosses” of 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 called “Experiments on Plant Hybridization” in

1865.

And Mendel corresponded with the influential Swiss botanist Carl Nägeli 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 biogenesis—Mendel had created a quantitative genetics.

And yet… nobody 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

experiments—because, 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 of “Mendelism,” 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 term “pangene”

for the unit of heredity, and for coming up with the term “mutation.”

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.

Also–also 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 called “Quantitative 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 the “father” of 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, Alfred “Hot Dog” Sturtevant, 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 order—of which genes come after which—not 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, they “reconstructed” their 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 information—in

an organism with way more chromosomes than fruit flies—was American geneticist Barbara

McClintock.

In the 1920s, she discovered how genes combine—and thus how information is exchanged when cells

divide.

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

to physical traits.

Then, in the 1940s and 50s, McClintock discovered transposition of genes, or the ability of

genes to change position on chromosomes.

She worked out how genes are responsible for turning physical characteristics on and off.

She explained color variation in corn, theorizing how genetic information is expressed across

generations, including why it's sometimes suppressed.

And yet McClintock stopped publishing her data in 1953 due to her colleagues' skepticism.

She was too far ahead of her time.

Basically, she got Mendeled.

But at least McClintock was awarded the Nobel in Physiology or Medicine in 1983—four decades

later—for her discovery of jumping genes.

She remains the only woman to receive an unshared Nobel Prize in that category.

Next time—we'll heat things up and get to work with the birth of thermodynamics!

Crash Course History of Science is filmed in the Dr. Cheryl C. Kinney studio in Missoula,

Montana and it's made with the help of all this nice people and our animation team is

Thought Cafe.

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