It all began with a single pea plant.
Humanity’s earliest experiments in heredity happened unconsciously, in sheep pens and wheat fields—transplanted grains that happened to thrive, short-legged sheep that conveniently stayed inside fences.
People selected from whatever traits emerged in each generation, but success came only through long waits and countless failures, arriving more by accident than by understanding.
When they tried to combine a “fiery-tempered meat hog” with a “small, gentle piglet” in hopes of creating the perfect animal, what they got instead were pigs that were both scrawny and irritable—one disappointing outcome after another.
People could see the results, but the pattern behind them remained invisible. They had no idea why good traits could vanish without warning, or why undesirable ones kept returning as if mocking their efforts.
01.The Science That Began in a Monastery Garden
Around this time, a lean, quiet priest—Gregor Mendel—planted a collection of pea plants in the backyard of St. Thomas Abbey, tucked away on the outskirts of the Austro-Hungarian Empire.
He wanted to push humanity’s long, confusing tradition of trial-and-error breeding away from “divine mystery” and toward something that could finally be explained by science.
Mendel set aside every established theory and returned to the facts. He gathered data—mountains of it—and tried to make sense of the patterns hidden within.
He chose traits that were measurable, visible, and cleanly distinct: tall vs. short, yellow vs. green, purple vs. white.
From their repeated patterns he forged a new idea—dominant and recessive traits.
Over eight years, and through more than eight thousand cross-bred pea seeds, Mendel discovered something remarkable:
green peas reappeared in the second generation; the same 3:1 ratio of traits surfaced again and again, as if obeying an unseen rule.
These patterns revealed a radical truth: heredity wasn’t a matter of fluids blending and fading.
It was built from “particles”—independent units that could separate and recombine in every new generation.
Figure 1. Results of the second pea cross
The ways in which genetic information can combine are virtually endless, yet the units themselves are remarkably resilient and stable. They can lie dormant for generations, only to reappear when the conditions are right.
In a sense, Mendel provided a solid, material foundation for Darwin’s theory, while Darwin gave Mendel’s discoveries a grand stage on which to demonstrate their significance.
02.What Exactly Are These “Particles” of Inheritance?
At first, Mendel simply called them “heritable factors.”
It wasn’t until the 20th century, when scientists dusted off Mendel’s old manuscripts, that his theory of particulate inheritance was rediscovered—and these heritable factors were given a new name: genes.
If Mendel’s peas in the monastery garden allowed humanity to glimpse the rules of inheritance for the first time, the generations of scientists that followed were determined to grasp the very essence of the gene.
They approached life from a chemical perspective. If genes were indeed particles, then surely they could be isolated and extracted, like any other substance.
They asked themselves: “If we could extract a particular substance from a yellow pea and introduce it into a green one to make it yellow, would this substance be the ‘yellow gene’?”
And so began a true chase for the gene itself—a quest that would reshape our understanding of life at the molecular level.
03.A Dead Mouse Reveals a Crucial Clue
Fred Griffith, while studying Streptococcus pneumoniae, stumbled upon a surprising clue.
He injected mice with a mixture of heat-killed “smooth” bacteria and live “rough” bacteria—a procedure that should have been the safest, most mundane experiment imaginable.
Yet the mouse died.
Even more puzzling, live smooth bacteria were found inside the dead mouse.
One mouse, one clue.
Griffith didn’t fully understand what had happened, but the result suggested something astonishing:
“Some form of ‘information’ had passed from the dead bacteria to the living ones, transforming them.”
Figure 2. Griffith’s pneumococcal transformation experiment
Oswald Theodore Avery at the Rockefeller Institute picked up the trail where Griffith left off.
He boiled smooth pneumococcal bacteria and systematically stripped away their components—lipids, proteins, polysaccharides—one layer at a time.
In the end, only a clear, fibrous substance remained.
This fibrous molecule had the chemical composition of DNA.
And crucially, the transformation effect disappeared only when enzymes that specifically degraded DNA were added.
Avery concluded that DNA was the substance carrying hereditary information.
From our vantage point today, we can easily cheer for Avery’s breakthrough.
He provided the sharpest explanation yet for the particulate inheritance Mendel had proposed nearly a century earlier.
But in the 1940s, the scientific community wasn’t ready to accept it.
“Perhaps,” they said,“you simply didn’t remove the proteins thoroughly enough.”
04.Viruses Deliver the Final Answer
After Thomas Hunt Morgan demonstrated through fruit fly experiments that genes reside on chromosomes, DNA once again surfaced in connection with heredity.
Coincidence—again?
American scientists Alfred Hershey and Martha Chase provided a decisive answer, using viruses as their tool.
They designed an elegant experiment: label DNA and proteins with different radioactive isotopes, let viruses infect bacteria, and then track which substance appears in the next generation of viruses.
The result was unmistakable:
it was the DNA that entered the offspring, not the protein.
At last, the true identity of the gene emerged.
Figure 3. The Hershey-Chase Experiment
05.The End of a Century-Long Pursuit, and the Start of Something New
From Mendel’s tiny particles of heredity, to Griffith’s perplexing results, to the confirmations by Avery, Hershey, and Chase, humanity spent nearly a century chasing down the truth:
“It is a strand of DNA.”
“It passes from one organism to another, determining traits.”
“It is the carrier of life’s blueprint.”
Yet this realization was not an ending.
On the contrary—it placed us at the true threshold of modern biology.
From this point on, the story of the gene finally began.
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