Hank Green: Today I have for you a sizzling tale of a chubby little friar who changed the world with a garden full of pea plants.

Today's great mind in science is Gregor Mendel, an Austrian monk who, in the mid 1800s, single-handedly fathered modern genetics. But even though Mendel was a man of God, as it were, he and his pea plants have been the focus of one of the biggest scientific scandals in history. Oh Lord, I love a good scientific scandal.

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Mendel was born in 1822, in what was then Austria but is now the Czech Republic. His parents were super super poor, but the young Gregor was such a precocious little scamp that they spent all of their money educating him. That is, at least until his dad was injured in a farming accident, which I imagine is the number one occupational hazard associated with being a pre-Industrial Revolution Austrian peasant. His family did what they could to help him out, but eventually Mendel decided that being an Augustinian monk didn't seem so bad.

Now, I'll point out here that the records of Mendel's life and work are pretty spotty, actually, because the priests who dealt with his personal effects after his death got rid of almost everything, not knowing how extraordinarily important it all was, but in one of the very few of Mendel's papers that survived, Mendel let on that he was not "called" to the church as some people were. "My circumstances decided my vocational choice," is how he put it.

Anyway, in 1843 Mendel moved into the monastery at Brno, Austria, and this turned out to be a pretty good move for him because he basically got sent to college for free, and, y'know, dating such a drag. Read that anyway.

After a failed attempt at becoming a science teacher, he started spending all of his free time in the gardens with some common garden peas, and, being a scientist at heart, he started doing experiments on them.

For the record, right now, you, and I don't care who you are, you know much much more about how heredity works than Mendel did when he started, not to mention chromosomes and DNA. At the time, microscopes weren't good enough to even observe basic cell division so nobody knew dog squat about how sexual reproduction worked. In Mendel's day, it was generally thought that making a baby was putting the parent's genetic material into a blender and just mixing it up real good. People assumed, for example, that if a white squirrel and a black squirrel were to have babies, their offspring would be gray.

What Mendel discovered after whispering sweet nothings to a yard full of pea plants for eight years was that this line of thinking was exactly entirely wrong. Mendel set us straight on the fundamental properties of inheritance, which eventually paved the way for development of modern genetics.

Mendel's choice of research subjects for this endeavor was shockingly perfect, for one important reason: the traits that he studied -- the color of pea flowers and the color and textures of the peas themselves -- are only determined by a single gene. This turns out not to be the case for almost every physical trait in most organisms. In fact, the vast majority of inherited traits are either the product of two or more genes working together to determine, say, eye color or ear shape, or the product of one trait having a hand in a bunch of different physical traits. How did Mendel know that? How did he know?

Well, he probably started out by noticing that the flowers of his pea plants in the garden were purple most of the time, but then every once in a while they produced white ones. Since he studied heredity in college, he knew that the way to get to the bottom of this was to create true breeding lines of purple flower peas and a true breeding line of white flower peas.

So he bred the purple ones together for successive generations, until he was getting purple flowers all of the time, and did the same thing for the white ones. Having done this, Mendel then started a series of extremely methodical experiments in which he bred a purely purple-flowered and the purely white-flowered plants together. And in doing this for successive generations, he eventually realized "Gott in Himmel, the pea flowers are white almost exactly one quarter of the time!"

This led him to three important conclusions.

Important conclusion #1: Mendel discovered that pea plants were inheriting a pair of genetic instructions from each parent. Sometimes, both instructions from a parent would tell the flower to be purple, sometimes they'd both be for white flowers, and sometimes there would be one instruction for each. Mendel called these versions of a gene passed from parent to offspring "factors", but these days we call them "alleles". And so the baby pea plant had two alleles for flower color, one chosen randomly from Mom and one chosen randomly from Dad, and these genetic instructions (the genotype, as we call it now) decided what the outward appearance of the flower (the phenotype) was going to be.

Important conclusion #2: Mendel also found that the allele for purple flowers was strong, or more dominant, than the white allele, which was recessive. Since the purple allele was dominant and the white was recessive, a plant inheriting one purple and one white allele would produce purple flowers.

Important conclusion #3: Even though the purple allele was dominant, that didn't mean that it was being tossed into the mix more often; it was just being expressed more often. In fact, Mendel concluded that which trait a parent was throwing into the ring (purple or white) was totally random, but a dominant allele was always going to trump a recessive allele.

So, though these three conclusions, Mendel came up with a hard and fast rule about genetic inheritance, Mendel's First Law, or the Law of Segregation, that says that every individual possesses two alleles for any particular trait (like, for example, flower color) and which allele a parent gives its offspring is completely random. The offspring then has one allele from Mom and one allele from Dad, and of those two alleles, the dominant one is the one that the offspring will express. If and when both of the alleles happen to be recessive, only then will the recessive trait be expressed.

But Mendel went even farther with his pea plants (and no, I'm not going to shut up about pea plants-- it's fascinating, okay?) and he got some more results in his experiments on the seeds of the pea plants, which are the peas. He discovered that two traits of the pea, its color and its skin texture, had nothing to with each other. Now, his peas could be either green or yellow in color, either have smooth or wrinkly skin. Mendel found that when he took a smooth yellow pea and crossed it with a green wrinkly pea, he could, with the same mathematical precision as he did with the flower, predict the ratio of yellow smooth, yellow wrinkly, green smooth, and green wrinkly peas.

The other rule that Mendel contributed to our understanding of genetics is Mendel's Second Law, or the Law of Independent Assortment, which says that separate genes are passed independently from each other from parent to offspring. In this case, two dominant traits in peas (the wrinkliness and the yellowness) were unrelated.

Pretty big deal, right? Well, Mendel ended up writing a paper called Versuche über Pflanzenhybriden [Experiments on Plant Hybrids], if you sprechen sie Deutsch, which clearly I do not, and that was published in a little rinketing scientific journal and he presented his findings to the equivalent of some 19th century garden clubs.

He also sent his paper to every fancy-pants scientists he could think of, but here's the thing: the big shots don't like to take notice of him because none of them knew what the hell he was talking about. Mendel's work was so far ahead of its time that his experiments didn't even make sense to his contemporaries. In fact, his data didn't become useful to researchers until nearly 35 years after he published them.

So, Mendel lived for another 20 years or so after he published his findings, and by all accounts he was a totally happy dude. He became the abbot of the monastery, and he had a lot of smart friends who liked to talk science. His doctor had him smoking 20 or so cigars every day to help him lose weight -- not an effective strategy, if anybody's interested -- but he was never recognized for the monolithic contributions he made to science during his lifetime.

But then, around 1900, scientists were independently working to figure out what Mendel had already discovered, because there were a lot of people still going around hollering, "Oh, white squirrel black squirrel make gray squirrel," which was by now becoming more and more obviously wrong. Microscopes had gotten a lot better and more powerful, and people were observing chromosomes. They had no idea what they were, but they were observing them. And it wasn't until a group of scientists dug up Mendel's papers and applied his laws and discoveries that had been made since that everybody working on heredity put down their beakers and were like, "Oh."

And suddenly the scientific community was totally bonkers for Gregor Mendel. He was heralded as the father of modern genetics and they made a little special shrine to him at his test garden at the monastery, and scientists would make pilgrimages there to weep over his hoes and shovels and stuff. Okay, I don't know about that last part, but the point is that suddenly Gregor Mendel became a very big deal.

And then, in 1936 the statistician named Ronald Fisher examining Mendel's data. Fisher was like, "Hey, you guys, um, I love Mendel and everything, but have you looked at his data? They're really good -- like, really good, like, maybe statistically implausible." Fisher was a statistician and a geneticist and a big Mendel fan, so in his research of Mendel's paper he admitted that in a lot of ways, Mendel really was the methodical genius superhero scientist that everyone gave him credit for being, but he also saw that all of Mendel's data corroborated his theory really well. Like, the data turned out to be eerily perfect for thousands of plants in dozens of experiments conducted over the eight years.

So Fisher was like, uh, "In conclusion, someone might have probably kind of fiddled with Mendel's data, but it definitely wasn't Mendel. Could have been one of his assistants, maybe?" Because, you know, Fisher hated to do it. He buried most of the really incriminating evidence about the data fiddling in the back of the paper, and some people read it, but most people didn't so it didn't cause much of a kerfuffle at first. But when the Mendel Centennial Celebrations came along in the 1960s, some people dug up Fisher's paper, and all of a sudden the scientific community went all animal planet on Mendel. Like, this guy, he existed before science, so we can't give him a super hard time about being like, "Well, then I must have crossed that pea wrong; don't include that one in the data." He didn't know about science! This was before real... this was before, like good science was being done. He was a monk. Give him a break!

Even now, there are Mendel-Fisher controversy aficionados out there who talk about this stuff 'til the lights go out. Nearly 50 years later, people are still writing books and papers about it, most of them trying to prove that Mendel was completely faultless, but nobody's been able to explain what Fisher found -- the apparent data fiddling, selective reporting, omission... whatever you wanna call it.

The important thing is that Mendel put us all on the right track, and of course genetics ended up being way, way way, way way way more convoluted than Mendel's work could explain, but what Mendel gave us was a firm hypothesis on which to base other studies by countless other geneticists who worked to discover and understand chromosomes and DNA in the 20th century.

Today, traits that have been shown to operate under Mendel's Laws, like albinism, are known as Mendelian traits. There are lists of these traits in humans and in other animals, and we've put some links to those down in the description.

If you have an idea for a great mind that you'd like us to profile, please let us know through the Twitters or the Facebooks, or of course in the comments below. If we pick your idea, I'll give you a shout-out in that episode -- either that or I can send you a Ziploc bag full of my shaved whiskers, whichever you want best.

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