Genetics - Lost and Found: Crash Course History of Science #25

Summary

This script tells the story of Gregor Mendel, the father of classical genetics, whose work on pea plants laid the foundation for understanding how traits are inherited. Despite his groundbreaking discoveries in the mid-19th century, Mendel's work was largely ignored during his lifetime. It wasn't until the early 20th century that his research was rediscovered and hailed as revolutionary, leading to the field of genetics as we know it today. The script also touches on the contributions of other scientists like Darwin, Galton, and the 'Fly Boys' who furthered our understanding of genetics.

Takeaways

• 🌱 Gregor Mendel, through his work with peas, is recognized as the father of modern genetics, despite his findings being overlooked during his lifetime.
• 🔍 Darwin's theory of evolution by natural selection explained the survival of traits but left the mechanism of inheritance a mystery, which Mendel's work later addressed.
• 🧬 Mendel's experiments with peas led to the formulation of the Laws of Mendelian Inheritance, including the Law of Segregation and the Law of Independent Assortment.
• 📊 Mendel's quantitative approach to genetics, using pea plants to track the inheritance of traits, was groundbreaking and provided a foundation for future genetic studies.
• 🏰 Mendel's isolation and lack of recognition can be attributed to his location in a remote abbey, his focus on practical plant breeding rather than theoretical science, and the scientific community's initial lack of interest in his work.
• 🌼 The rediscovery of Mendel's work around the turn of the 20th century by four different researchers, including Hugo de Vries, Carl Correns, Erich von Tschermak, and William Jasper Spillman, reignited interest in genetics.
• 🧪 The Fly Room experiments at Columbia University by Thomas Hunt Morgan and his team, using fruit flies, further solidified Mendelian genetics and led to the development of genetic linkage maps.
• 🏆 Thomas Hunt Morgan was awarded the Nobel Prize for his work on chromosomes and heredity, highlighting the importance of Mendelian genetics in understanding biological inheritance.
• 🌽 Barbara McClintock's discovery of genetic transposition in corn, or 'jumping genes', expanded upon Mendelian genetics by showing how genes can change position and affect trait expression across generations.
• 📚 The development of model organisms like fruit flies, corn, and others has been crucial for genetic research, allowing scientists to study inheritance and mutation in controlled environments.

Q & A

Who are some scientists that were recognized for their revolutionary work during their lifetime?

Darwin and Pasteur were scientists who were recognized for their revolutionary work during their lifetime and became massive celebrities.

What was Charles Darwin's theory on how organisms adapt to their environment?

According to Darwin, organisms have slightly different traits, and these variations become more important over time as environments change, making some traits more useful than others.

What was Darwin's hypothetical unit of heredity called?

Darwin called the hypothetical unit of heredity the 'pangene,' which is the origin of the term 'gene.'

Who was Gregor Mendel and what was his contribution to the field of genetics?

Gregor Mendel was a contemporary of Darwin who conducted experiments with English peas and is now known as the 'father of genetics.' He discovered the principles of inheritance, including dominant and recessive traits.

What were the three traits Mendel focused on in his pea plant experiments?

Mendel focused on seven traits in his pea plant experiments: seed color, individual seed shape, unripe seed pod color, seed pod shape, flower color, flower location, and plant height.

What did Mendel coin the terms for to describe certain traits that were passed down?

Mendel coined the terms 'dominant' and 'recessive' to describe traits that were passed down in his pea plant experiments.

What are Mendel's Laws of Inheritance?

Mendel's Laws of Inheritance include the Law of Segregation, the Law of Independent Assortment, and the Law of Dominance, which describe how genes controlling traits are distinct, how genes for different traits assort independently, and how some traits are dominant over others.

Why did Mendel's work initially go unnoticed by the scientific community?

Mendel's work went unnoticed initially because he was not focused on promoting a grand theory of life, he was isolated geographically, had bad luck with his second model plant, became busy with administrative duties after a promotion, and was scientifically ahead of his time.

Which four researchers rediscovered Mendel's work around the turn of the 20th century?

The four researchers who rediscovered Mendel's work were Dutch botanist Hugo de Vries, German botanist Carl Correns, Austrian agronomist Erich von Tschermak, and American economist William Jasper Spillman.

What was the significance of the 'Fly Boys' in the history of genetics?

The 'Fly Boys,' led by Thomas Hunt Morgan at Columbia University, conducted extensive experiments on fruit fly genetics, leading to the development of genetic linkage maps and the establishment of Drosophila melanogaster as a model organism.

Who was Barbara McClintock and what was her major contribution to genetics?

Barbara McClintock was an American geneticist who discovered transposition of genes, or the ability of genes to change position on chromosomes. She explained color variation in corn and how genetic information is expressed across generations.

Outlines

00:00

🌱 The Legacy of Gregor Mendel

This paragraph introduces the story of Gregor Mendel, the father of modern genetics, whose work was initially overlooked but later recognized as foundational. It discusses Darwin's theory of natural selection and the concept of 'pangenes' as a precursor to the modern understanding of genes. Mendel's background as a monk and his scientific curiosity in breeding plants, particularly peas, led to his groundbreaking experiments. His work on the inheritance of traits through dominant and recessive genes and the formulation of the Laws of Mendelian Inheritance are highlighted. Despite the significance of his findings, Mendel's work was not appreciated during his lifetime, and he faced challenges in replicating his pea plant experiments with hawkweed, which reproduces asexually.

05:01

🔬 Rediscovery and Advancement of Genetics

The second paragraph details the rediscovery of Mendel's work around the turn of the 20th century by four researchers: Hugo de Vries, Carl Correns, Erich von Tschermak, and William Jasper Spillman. Each played a role in promoting 'Mendelism' and establishing genetics as a field of study. The paragraph also discusses the controversy between blending inheritance and the discontinuous inheritance supported by Mendel's work. The 'Fly Boys' at Columbia University, led by Thomas Hunt Morgan, are credited with advancing genetics through their work on fruit flies, which included the development of genetic linkage maps by Alfred Sturtevant. The use of model organisms and the contributions of Barbara McClintock in玉米基因研究 and the discovery of transposable elements are also mentioned, highlighting her delayed recognition and eventual Nobel Prize.

10:06

🏆 Triumph and Recognition in the Field of Genetics

The final paragraph focuses on the later recognition and accolades within the field of genetics. It emphasizes Barbara McClintock's discovery of 'jumping genes' and her Nobel Prize in Physiology or Medicine in 1983, noting her as the only woman to receive an unshared Nobel in that category. The paragraph concludes with a teaser for the next topic, thermodynamics, and provides information about the production of Crash Course, including the studio location, the animation team, and the support from patrons on Patreon.

Keywords

💡Gregor Mendel

Gregor Mendel is known as the father of modern genetics. He was an Austrian monk and scientist who conducted experiments on pea plants to understand how traits are inherited. His work, though initially overlooked, laid the foundation for the field of genetics. In the video, Mendel's experiments with pea plants led to the discovery of dominant and recessive traits, which is a fundamental concept in understanding heredity.

💡Darwin

Charles Darwin is a central figure in the history of science, known for his theory of evolution by natural selection. The video mentions Darwin in contrast to Mendel, highlighting that while Darwin understood the importance of variation in traits, he did not know the mechanisms of inheritance. Darwin's concept of 'pangenes' was a precursor to the modern understanding of genes.

💡Traits

Traits refer to the characteristics or features of an organism that can be inherited from one generation to the next. In the context of the video, Mendel's work focused on identifying and understanding how specific traits, such as seed color or plant height, are passed down through generations.

💡Dominant and Recessive Traits

These terms describe the way certain traits are expressed in offspring. Dominant traits are those that are expressed when one or both copies of a gene for that trait are present, while recessive traits are only expressed when two copies of the gene are present. Mendel's work with pea plants helped to clarify these concepts, as he observed how traits like seed color or plant height were passed down.

💡Laws of Mendelian Inheritance

These are the fundamental principles of heredity that Mendel discovered through his pea plant experiments. The video mentions three laws: the Law of Segregation, the Law of Independent Assortment, and the Law of Dominance. These laws explain how traits are inherited and how they can appear or disappear in successive generations.

💡Heredity

Heredity is the passing of traits or characteristics from parents to their offspring. The video discusses how Mendel's work was crucial in understanding the mechanisms of heredity, which was a mystery during Darwin's time. Hereditary science, or genetics, is now a cornerstone of biology.

💡Genetics

Genetics is the study of genes, heredity, and the variation of organisms. The video explains how Mendel's work was the birth of classical genetics, a field that later expanded to include the study of DNA and the complex interactions of genes within organisms.

💡Carl Nägeli

Carl Nägeli was a Swiss botanist mentioned in the video as someone who corresponded with Mendel. Nägeli's work and his connection to Mendel highlight the scientific community's engagement with the ideas of heredity and plant breeding during Mendel's time.

💡Hawkweed

Hawkweed is a plant that Mendel attempted to use to reproduce his pea plant experiments. The video explains that hawkweed reproduces asexually, which led to Mendel's inability to replicate the patterns he observed in peas. This example illustrates the challenges Mendel faced in validating his theories.

💡Rediscovery of Mendel

The video discusses how Mendel's work was not recognized during his lifetime but was rediscovered around 1900 by several researchers independently. This rediscovery led to a renewed interest in Mendel's theories and the establishment of genetics as a scientific discipline.

💡Model Organism

A model organism is a species that is extensively studied to understand particular biological phenomena. The video mentions fruit flies (Drosophila melanogaster) as a model organism used in genetic research. Model organisms allow scientists to study genetics and other biological processes in a controlled and reproducible manner.

Highlights

Darwin and Pasteur were known for their revolutionary work during their lifetimes.

Gregor Mendel's work on genetics was not recognized until decades after his death.

Darwin proposed the concept of 'pangenes' as units of heredity, which later evolved into the term 'gene'.

Mendel, a contemporary of Darwin, conducted extensive experiments with pea plants to study inheritance.

Mendel's work focused on seven distinct traits in peas, which he found to be inherited independently.

Mendel introduced the terms 'dominant' and 'recessive' to describe how traits are passed down.

His experiments led to the formulation of the Laws of Mendelian Inheritance.

Mendel's findings were initially ignored, partly due to his isolation and lack of promotion.

Mendel's work was rediscovered around 1900 by four different researchers, leading to the field of genetics.

Hugo de Vries, Carl Correns, Erich von Tschermak, and William Jasper Spillman independently recognized Mendel's contributions.

The rediscovery of Mendel's work led to a new understanding of heredity and the establishment of genetics as a science.

The 'Fly Boys' at Columbia University, led by Thomas Hunt Morgan, furthered genetics through fruit fly experiments.

Alfred Sturtevant developed genetic linkage maps, which helped locate genes on chromosomes.

The Fly Room's work resulted in the foundational textbook 'The Mechanism of Mendelian Heredity'.

Barbara McClintock's research on corn led to the discovery of transposition of genes.

McClintock's work on 'jumping genes' was initially met with skepticism but later earned her a Nobel Prize.

Crash Course History of Science is a production of Complexly, aiming to educate on scientific discoveries.

Transcripts

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!

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