If I were to say “loving plants is in my DNA,” you’d know what I meant, right?

Plants are one of my major interests —they’re integral to my identity and a huge part of my life. I even mean it sort of literally because my mom passed down her love of plants to me.

Well, believe it or not, the average person knowing about DNA is a pretty recent thing. Scientists only finished mapping all the DNA that makes a human in 2022. And that breakthrough built on more han 150 years of genetics research, studying how traits are passed from generation to generation.

Long before we knew about double-helixes or cloned sheep, a monk figured out some of the defining characteristics of genetics using an unlikely pair of tools — peas and a paintbrush. Hi, I’m Alexis, and this is Crash Course Botany. [THEME MUSIC] Let me introduce you to a guy named Gregor Mendel, born in 1822 in what is now the Czech Republic. Mendel grew up to be a monk, and he used the garden at his monastery to study plants.

But that didn’t look like reading a textbook or staring through a microscope. For Mendel, it looked more like plant matchmaking. In his most famous experiments, he hybridized pea plants, or interbred plants with different characteristics.

He did this by moving pollen from one plant to another to manually fertilize them — which works because pollen produces sperm cells. And while farmers had been cross-breeding plants and animals for hundreds of years, no one had yet found a reliable, scientific way to control the color, taste, texture, or other characteristics of organisms — what we call, in genetics, traits. At the time, most people thought that traits in offspring were a blend of the parents’ traits, assuming, for example, that one tall parent and one short parent would yield a child of medium height.

But Mendel’s experiments — and my shockingly tall sister — would prove this assumption all wrong. And pea plants were the perfect subject for this research. They’re relatively quick and easy to grow, and it’s pretty simple to control their reproduction because they’re able to both self-pollinate and cross-pollinate —meaning they can use their own pollen or the pollen of another plant to reproduce.

They also have clear and distinguishable traits that are easy to trace through different generations. Telling the difference between green peas and yellow peas is a lot easier than trying to classify and track the many varied hair colors in people, for example. I mean, I still have no idea if my third-grade bestie had brown or dirty blonde hair.

So, Mendel conducted a bunch of experiments. He pollinated more than ten thousand plants over the course of eight years. To say he was as busy as a bee is… maybe the most literal interpretation of that phrase ever.

But let’s see him in action in the Thought Bubble… Mendel’s first step was observation. For two years, he monitored pea plants as they self-pollinated. And in self-pollination, traits were consistent from generation to generation— tall plants made more tall plants, short plants made more short plants, and so on.

Each category had two possibilities: pods could be yellow or green, seeds could be smooth or wrinkly, and so on. And later, Mendel studied other traits, like flower color. Once he knew that these traits were all constant in his set of plants, he began to cross-breed plants with different traits—by dusting pollen from one plant onto another with a paintbrush.

You might say it was an art and a science. What Mendel found was astonishing. Rather than blending — like how purple and white paint can make lavender— genetics appeared to favor the traits of one parent over the other.

In other words, a purple-flowered plant and a white-flowered plant didn’t produce offspring with lavender flowers; they produced a plant with regular purple flowers. And then in the next generation, “hidden” traits could crop back up again— two purple-flowered plants might produce a plant with white flowers. So, crossing two plants with the same traits could yield completely different-looking plants apparently out of nowhere.

Thanks, Thought Bubble! Though he didn’t know it, Mendel was observing genetics in action —discovering the basics of how traits are passed down in way more than just pea plants. The field of genetics has grown enormously since those early experiments, and we’ve developed all kinds of special words to describe what Mendel was doing—language we didn’t know we needed before.

So today, we call the observable traits and behavior of a living thing its phenotype. All the plants Mendel studied showed their phenotype in the traits he recorded— the yellow or green peas, the bumpy or smooth seed pods, the tall or short plants. And the differences in phenotypes are controlled by the underlying instructions inside of cells.

Since the instructions are carried in packets called genes, this is known as the genotype. While Mendel didn’t know about genes, he was observing how those internal instructions were changing the plants season after season. And by collecting careful data about the phenotypes of the plants and using statistics to interpret the results, Mendel figured out what was going on even without that knowledge of genes.

We call Mendel’s main ideas his three Principles of Inheritance. The first, called the Principle of Dominance, describes how the genes that control the plant's phenotype actually work. Basically, every cell in the pea plant contains two copies of a gene for a particular trait, one from each parent plant.

Because of this, we say that it’s a diploid organism. That is going to be important in a moment. These copies of a gene can come in different varieties, known as alleles.

So, for the pea shape characteristic, there’s an allele for smooth, and an allele for wrinkled. Out of those two alleles, one is dominant, and the other is recessive, which are terms Mendel himself came up with. In this case, the allele for a smooth texture is dominant, and the wrinkled shape is recessive.

So whenever the dominant allele is present in the pair, it will always be shown in the phenotype, even if the recessive allele is there as well. When Mendel saw a smooth pea, he knew it could have either two smooth alleles, or a smooth and a wrinkled allele. But when he saw a wrinkled pea, he knew it had to have two wrinkled alleles, because if there’d been a smooth allele there, the pea would have been smooth.

Dominant alleles are sort of like that person at the party who’s shouting louder than their friends, so they’ll always be heard. And the recessive trait is more like the soft-spoken person you’ll only hear if their loud-mouth friend is out of the room. Okay, so the next idea he came up with was the Principle of Segregation, which describes how these alleles are passed on from parents to offspring.

Most of the cells in a pea plant are diploid, but the plant also makes eggs and sperm, known as gametes, which are the cells involved in reproduction. They’re made from splitting normal diploid cells to separate the alleles, so the egg or sperm cell ends up with only one copy of each gene —that way they can each give one to the offspring. We say that these cells are haploid, because they have half the normal number of alleles.

So, if a plant starts out with both kinds of alleles for pea shape, some gametes end up with the smooth allele, and some have the wrinkled one. Then, when the sperm and egg come together in fertilization, the gametes each contribute one allele to making a new pair. And these alleles are combined randomly, so there’s no telling which combination or which phenotype the new seed will end up with.

That’s how you can sometimes end up with a surprising trait. Two parents could both have a dominant and a recessive allele, and therefore show only the dominant phenotype. But if their two recessive alleles come together in the offspring, they’d get expressed in its phenotype.

Finally, Mendel’s Principle of Independent Assortment describes how every allele is split up from its partner independently of all the other alleles in the genetic code. So whether the sperm ends up with a smooth or wrinkled pea shape allele has absolutely no effect on whether it ends up with a green or yellow pea color allele. Each allele has an equal opportunity to hang out with every other one— they can mingle all they want at this party!

And because of the sheer number of genes it takes to make a typical plant, this random shuffling helps to create the astonishing variety of unique phenotypes in plants today. So, you’d think these discoveries would have rocked the scientific world at the time, right? Well, not exactly.

Mendel published his findings about dominant and recessive genes and presented them to other scientists. But they didn’t make much of a splash, possibly because his ideas were so different from what the scientific community believed about heredity. And a couple years later, Mendel became head monk at his monastery, and he didn’t have much time for botany anymore.

It wasn’t until 1900—sixteen years after Mendel died— that several scientists unearthed his paper and realized just how groundbreaking it was. After that, the ripple effects for the study of genetics were immediate. Because, it turns out that Mendel’s principles of inheritance don’t just apply to pea plants, or even just to plants.

This is, on a basic level, how all life works, from the tiniest insect to the largest blue whale. Today, scientists know that our genes are actually encoded in our DNA, the incredibly long molecules found in pretty much every cell. And we know that not all traits are as straightforward as the color or shape of peas.

Many organisms—including other plants—have traits with far more possibilities. Like, how much rice is produced by a rice plant involves how much the grains weigh, how many flower clusters they have, and the number of grains per flower cluster. Or take eye color in humans – you can have blue, green, brown, hazel, or gray eyes, and many colors in between.

These are known as complex traits, and the variation in phenotype comes from having several different genes contributing to them and, sometimes, environmental or behavioral factors as well. And we’ve learned that sometimes genes are linked together, so independent assortment doesn't always apply. Thanks to Mendel’s ideas, we also know now that these complicated DNA molecules are subject to change as they’re copied over and over again.

Ultimately, these mistakes result in mutations in the genetic code that can create brand new alleles or traits that never even existed before. And mutations can have different effects on an organism. Like, one species of monkey flower developed a mutation that changed the color of its petals.

The new red petals attracted hummingbirds instead of bees, and opened up new possibilities for pollination. So this was an advantageous, or helpful, mutation. On the flip side, there are deleterious mutations, which are unhelpful for survival — like some that cause corn plants to produce less corn.

And you wouldn’t even know if a neutral mutation occurred in a plant’s DNA ‘cause it would still look and act the same. So, mutations, together with the general principles of inheritance, help to drive evolution — in plants and in every other living thing on Earth. Today, we still learn about and use these so-called Mendelian genetics because the system does a pretty good job of explaining the basics of inheritance and variation.

The astonishing diversity of plant life all over our green planet is the result of millions of generations of plants trading alleles according to Mendel’s principles. Now, his principles don’t cover all genetic scenarios — and if you want to dig a little deeper, you can explore some controversy around his original results. But at the end of the day, this research laid the groundwork for all of modern-day genetics.

Without those pea plants, we would know a whole lot less about the world—and ourselves. Next time, we’ll focus on the incredible modern-day feats of plant genetics, from selective breeding to genetic modification. Hey foodies, you’re missing out if you’re not watching the new stuff cooking over on PBS Food.

If you’re into learning about diverse food recipes, check out Pan Pals and a bunch of other shows that range from the history of iconic dishes, to food science. Hit the link in our description to check them out. Disclaimer, don’t watch these on an empty stomach!

Hey, before we go, let’s branch out! What plant did Mendel study after his famous pea plants? Weed through the comments to find the answer!

Thanks for watching this episode of Crash Course Botany which was filmed at the Damir Ferizović Studio and made in partnership with PBS Digital Studios and Nature. If you want to help keep Crash Course free for everyone, forever, you can join our community on Patreon.