Genetics 101 with Mendel

Lesson 1

Mendel in the monastery garden

Gregor Mendel was born in 1822 in a farming village in Silesia, in what is now the Czech Republic, and he was christened Johann. Money was short, his schooling nearly ended early, and the path that saved his education was an unlikely one: in 1843 he entered the Augustinian monastery of St Thomas at Brunn, took the name Gregor, and found there a library, a scientific culture, and time to think. The monastery was no retreat from learning. Its monks taught in local schools, studied plants and weather, and kept a garden where careful experiments were welcome.

Mendel taught physics and natural history at the town school, studied for a time in Vienna, and came home with a question that breeders and naturalists had chewed on for generations without an answer. When two different varieties are crossed, what exactly happens to their contrasting characters in the children, the grandchildren, and beyond? Earlier answers were mostly impressions. Mendel decided to answer with arithmetic. Starting in 1856, in a small monastery garden plot, he began crossing varieties of the garden pea, and he kept at it for about eight years, growing tens of thousands of plants and counting their offspring one pod at a time.

He read his results to the local natural history society in 1865 and published them in 1866 in the society journal, under a modest German title about experiments in plant hybridisation. The paper is a model of clarity: the plan, the counts, and the rule he drew from them, all laid out so that anyone could repeat the work. Almost nobody did, at first. In 1868 Mendel was elected abbot of the monastery, and administration, visitors, and a long tax dispute with the government swallowed the time that science had used. He died in 1884, known locally as a kind abbot and a keen beekeeper and gardener, with his great paper sitting unread in libraries across Europe. Lesson 6 tells how it was found again.

Lesson 2

Peas, traits, and careful counting

Good experiments begin with a good subject, and Mendel chose his with care. The garden pea, Pisum sativum, fertilises itself inside a closed flower, so a variety left alone breeds true, generation after generation. It is also easy to cross on purpose: open the flower before it matures, remove its pollen parts, and dust in pollen from another plant with a small brush. Peas grow fast, take little room, and offer sharp either-or characters with no confusing middle ground. As Mendel put it in his paper, in a line that should hang over every laboratory bench: “Transitional forms were not observed in any experiment.”

He settled on seven pairs of contrasting characters, each easy to score at a glance. Round seeds against wrinkled, angular ones. Yellow seed colour against green. Grey-brown seed coats with violet-red flowers against white coats with white flowers. Smooth, inflated pods against pods pinched in between the seeds. Green unripe pods against yellow ones. Flowers spread along the stem against flowers clustered at the top. And tall stems, six or seven feet long, against dwarf stems of a foot or so. Before the real crosses began, he spent two years checking that each variety bred true for its character, so that any surprise later would mean something.

Then came the counting, and this is where Mendel left every predecessor behind. He did not record that a cross gave mostly round seeds. He recorded that 253 hybrid plants gave 7,324 seeds, of which 5,474 were round or roundish and 1,850 were angular and wrinkled. He counted seed colour across 8,023 seeds from 258 plants. He grew more than a thousand plants just to score stem length. Big numbers were the point: in small samples, chance wobbles wildly, but in thousands of offspring a hidden ratio can stand up and be seen. Every later lesson in this course runs on those notebooks.

Lesson 3

Dominant and recessive

Cross a pure round-seeded pea with a pure wrinkled one and you might expect a blend, seeds slightly less round, the way mixing paints gives a middle colour. That is not what happened. Every seed of the first hybrid generation, the generation we now call F1, was round. The wrinkled character had not been weakened or diluted. It had vanished completely, and yet, as the next lesson shows, it was not destroyed. Mendel needed words for this behaviour, and the two words he coined are still on every genetics page today.

Here is his own definition, in William Bateson's translation: the characters “which are transmitted entire, or almost unchanged in the hybridisation” are termed the dominant, and those “which become latent in the process” are the recessive. Round is dominant over wrinkled. Yellow is dominant over green. Tall is dominant over dwarf. In each of his seven pairs, one character completely masked the other in the hybrid. Mendel also checked that it made no difference which parent supplied the pollen and which supplied the seed. Reciprocal crosses gave identical hybrids, a quiet result with a loud meaning: mother and father contribute equally to these characters.

Hold on to the word latent, because it carries the whole discovery. The recessive character in a hybrid is present but hidden, carried along silently, able to reappear untouched in a later generation. Heredity, in other words, is not blending ink. It behaves like a deck of separate cards that can be shuffled, dealt, hidden in a hand, and played again later exactly as they were. Mendel did not know what the cards were made of. Nobody would know for decades. But he had established that they exist, that they come in pairs, and that one of each pair can mask the other without changing it. That is the foundation everything else in this course stands on.

Lesson 4

The 3:1 ratio

Now let the hybrid plants fertilise themselves, as peas naturally do, and grow the next generation, F2. The vanished recessive comes back. Wrinkled seeds appear again among the round, green among the yellow, dwarf plants among the tall, and always in roughly the same proportion. Mendel's summary deserves quoting exactly: among each four plants of this generation “three display the dominant character and one the recessive.” Not about half. Not a vague majority. Three to one, for every one of the seven characters, without exception.

Look at his actual counts and watch how close they land. Seed form: 5,474 round to 1,850 wrinkled, a ratio of 2.96 to 1. Seed colour: 6,022 yellow to 2,001 green, a ratio of 3.01 to 1. Stem length: 787 tall to 277 short, a ratio of 2.84 to 1. Pod form, pod colour, seed coat colour, and flower position all fell between 2.82 and 3.15 to 1. Averaged across all seven experiments, Mendel reported 2.98 to 1, or, as he rounded it, 3 to 1. Try it yourself in miniature: toss the counts into groups of four hundred offspring and you would expect about three hundred showing the dominant character and about one hundred showing the recessive.

The deepest part of the discovery hides inside that three. Mendel grew the F2 plants on for another generation and found that the dominant-looking three were not all alike. One in three bred true, giving only dominant offspring forever. The other two behaved exactly like the original hybrids, splitting again into three dominant to one recessive. So the visible 3:1 is really a hidden 1:2:1: one pure dominant, two hybrids, one pure recessive. The recessive quarter, meanwhile, bred perfectly true from then on. A ratio that anyone can count had revealed an invisible structure underneath, and Mendel saw it because he refused to stop counting after one generation.

Lesson 5

Two traits at once

One character at a time is tidy, but real organisms carry thousands of characters together. Mendel asked the obvious next question: if you cross plants differing in two characters at once, do the characters drag each other along, or does each keep its own 3:1? He crossed a variety with round, yellow seeds against one with wrinkled, green seeds. The F1 seeds were all round and yellow, both dominant characters winning at once, exactly as expected. Then he let those hybrids self-fertilise and counted all 556 seeds they produced, sorting each seed into one of four piles.

The piles came out like this: 315 round and yellow, 108 round and green, 101 wrinkled and yellow, and 32 wrinkled and green. Compare those counts with the famous ratio 9:3:3:1. Out of 556 seeds, that ratio predicts about 313, 104, 104, and 35. Mendel's real piles sit almost on top of the prediction. And the 9:3:3:1 is no new magic: it is simply the 3:1 multiplied by itself. Three quarters round times three quarters yellow gives nine sixteenths round and yellow. Each character assorts on its own, independently of the other, so the combined pattern is just the two single patterns multiplied together.

Mendel confirmed the point by growing the seeds on and checking that each pile contained the hidden mixtures his theory required, pure forms and hybrids in just the proportions the multiplication predicts. He even ran a three-character experiment with the same result. This principle, that character pairs assort independently, is now called Mendel's second law. One honest footnote, which Mendel could not have known: it holds cleanly when the factors for the two characters sit on different chromosomes, or far apart on the same one. Factors sitting close together on the same chromosome can travel together, a complication called linkage that later geneticists, working with fruit flies, would map in beautiful detail. Mendel, by luck or by judgment, had picked pea characters that mostly play by the simple rule.

Lesson 6

Forgotten for 35 years, then rediscovered

Mendel sent his paper into the world in 1866 and the world shrugged. The journal that printed it reached over a hundred libraries, and Mendel mailed reprints to leading botanists, but the paper was cited only a handful of times in 35 years. Partly it was the maths: biologists of that era described and classified, and a paper full of ratios looked like physics envy. Partly it was timing: the cell structures that would explain his factors, the chromosomes, were only just being seen under microscopes, and nobody yet connected them to heredity. Mendel himself moved on to bees, weather records, and running his monastery, and when he died in 1884 his obituaries praised a beloved abbot, not a scientist.

Then, in a single spring, 1900, three botanists in three countries, Hugo de Vries in Holland, Carl Correns in Germany, and Erich von Tschermak in Austria, each published results matching Mendel's ratios, and each, to their credit, pointed readers back to the forgotten paper they had found in the literature. Heredity finally had its founding document. In England, William Bateson became its champion: he translated Mendel's papers, defended them against fierce critics in his 1902 book Mendel's Principles of Heredity: A Defence, the very book this course is built on, and soon after gave the new science its name, genetics. Mendel's monastery garden became a place of pilgrimage.

One honest wrinkle belongs in this lesson, and this course will not hide it. In 1936 the statistician R. A. Fisher reanalysed Mendel's published counts and found them, overall, a little too close to the expected ratios, closer than pure chance would usually deliver across so many experiments. Historians and scientists still debate what that means: unconscious tidying, an assistant who knew the expected answers, Mendel stopping counts when the numbers looked right, or Fisher's own assumptions being too strict. Nobody sensible claims the laws are wrong: the ratios have been repeated, with normal wobble, in countless organisms ever since, and the 1:2:1 structure underneath them is real. Read the debate as a lesson in itself. Even a founding hero's notebooks deserve checking, and good science survives the checking.

Lesson 7

Punnett squares by hand

In the years after the rediscovery, the Cambridge geneticist Reginald Punnett popularised a pencil-and-paper grid that turns Mendel's ratios into something you can work on a scrap of paper. Geneticists still call it a Punnett square. You need only Mendel's rules: each parent carries two copies of a factor, the copies separate when sex cells form, and each offspring receives one copy from each parent. Use a capital letter for the dominant factor and the matching small letter for the recessive: T for tall, t for dwarf.

Work the classic cross by hand. Take two hybrids, Tt crossed with Tt. Draw a two by two grid. Across the top write the sex cells of one parent, T and t. Down the side write the sex cells of the other parent, T and t. Now fill each box by combining its row and column letters. The four boxes read TT, Tt, Tt, and tt. Read off the constitutions: one TT, two Tt, one tt, Mendel's hidden 1:2:1. Now read off what the plants look like: TT and both Tt plants are tall, only tt is dwarf, so three tall to one dwarf. You have just derived the 3:1 ratio instead of memorising it, and it will work for any of Mendel's seven characters.

Push the grid one step further. Cross Tt with a dwarf, tt, the test cross breeders use to expose a hidden recessive. The grid now gives two Tt and two tt, a 1:1 split of tall to dwarf, and any dwarf offspring proves the tall parent was a hybrid. For two traits, the same method grows to a four by four grid, sixteen boxes, and if you fill in a cross of two double hybrids, RrYy with RrYy, and sort the boxes by appearance, you will count nine round yellow, three round green, three wrinkled yellow, and one wrinkled green. Sixteen boxes on paper reproduce the 556 seeds Mendel counted in his garden. Do one square yourself, slowly, before you move on. This is a course you can literally redo by hand.

Lesson 8

From Mendel to DNA

Mendel called his invisible units factors, and for forty years nobody knew what a factor physically was. The answer assembled itself in stages. Biologists watching dividing cells saw thread-like bodies, chromosomes, halving themselves exactly as Mendel's separating factors should when sex cells form. In the 1910s Thomas Hunt Morgan's fruit fly laboratory showed that factors, now called genes, sit in a line along the chromosomes, and that their positions could be mapped by how often characters were inherited together. Mendel's first law, the separation of a pair of factors into different sex cells, turned out to be a direct picture of what chromosomes do in the division called meiosis.

The final identification came in 1953, when the structure of DNA was worked out: a double helix whose sequence of chemical letters spells the genes. A dominant and a recessive version of a gene, like Mendel's round and wrinkled, are the same stretch of DNA with a difference in the spelling. The wrinkled pea, we now know, carries a disrupted gene for an enzyme that processes starch, so its seeds hold more sugar, take in more water as they ripen, and collapse into wrinkles as they dry. Mendel's abstract factor became chemistry you can read letter by letter, and his ratios became predictions about how those letters are dealt to children.

Everything in modern genetics is a footnote to the monastery garden. Medical genetics traces disease genes through families using Mendel's rules, and a Punnett square still gives a couple their odds for a recessive condition. Plant and animal breeders plan crosses with his ratios, now accelerated by DNA testing that reads the factors directly instead of waiting a generation to see them. The honest closing note is that real heredity is richer than seven pea traits: many genes influence most characters, environment matters, and linked genes bend the simple ratios. But the foundation has never moved. Heredity comes in separate, stable units, passed whole from parent to child, and a patient monk counting peas was the first person to prove it.

Sources

The Book Behind This Course

This course is taught from one book, free for anyone to read in full at Project Gutenberg. It contains William Bateson's defence of Mendel together with Bateson's translations of Mendel's own papers, so Mendel speaks in these lessons in his own words.

• Mendel's Principles of Heredity: A Defence, by William Bateson, with translations of Gregor Mendel (1902), Project Gutenberg

Graduation

You finished Genetics 101

You stood in the monastery garden at Brunn, counted seeds by the thousand, watched a vanished trait return in a perfect three to one, multiplied two ratios into nine to three to three to one, and worked the grid that still bears Punnett's name. You also heard the hard questions honestly, including Fisher's, and saw why the laws survive them. Congratulations: you can now read a family tree, a seed catalogue, or a genetics headline with Mendel's eyes.

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