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 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
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