"Facts do not cease to exist because they are ignored ." --Aldous Huxley

I. Once upon a time, there was a monk, living in a monastery in what was then the town of Brünn in the Austro-Hungarian Empire. (The town is now in the Czech Republic, and called Brno. . .)

1. This monk, Brother Gregor Mendel by name, had studied at the University of Vienna when he was younger, but had flunked out after two years. That did not, however, keep him from an interest in botany -- he was a farmer's son, after all.
2. As a monk, he taught at the monastery's high school. Now and then he did some research, on agricultural pests, beekeeping, and weather.
3. He also wondered about the problem of heredity, a problem that had the best minds in Europe stumped.
4. Mendel managed to get permission from the Abbot (the head of the monastery) to do some experiments on plant breeding, in the monastery garden.
5. Through an amazing stroke of luck, he chose to work on pea plants.
   1. Like all flowering plants, peas produce seeds when the female parts of their flowers (which contain eggs) receive pollen (which contain sperm).
      1. Normally, pea flowers are completely enclosed by their petals at the time they are pollinated. The result is that each flower fertilizes itself.
      2. Because pea flowers fertilize themselves, normally each pea plant produces seeds that grow into plants that look just like the parent plant -- the peas "breed true".
      3. A true-breeding plant with purple flowers, for instance -- if it's left alone to self-fertilize -- will make seeds that all grow into purple- flowered plants. This will continue for as many generations as you care to grow the peas.
   2. Another advantage of using peas is that there were -- and are -- several different strains of cultivated pea plant, with distinctive features that are easily told apart.
      
      Top: Purple-flowered peas and white-flowered peas.
      Bottom: Inflated-pod peas and constricted-pod peas.
      These are four of the 14 strains of pea that Mendel used in his experiments.

   3. Mendel's experiments involved creating hybrids -- crosses between two different strains.
      1. Brother Gregor could fertilize each flower artificially -- by cutting the petals open, using a small paintbrush to put pollen from another plant on the female parts of the flower, and then sealing the petals closed again with a bit of wax.
      2. This meant that he could carefully control which pea plants fertilized which. (An open flower could be pollinated by pollen from a random plant blowing in the air.)
   4. And there were other advantages: peas didn't take up much room to grow, they grew quickly, and you could eat them when your experiment was done.
6. Gregor Mendel began his work in 1856.
   1. He experimented with crossing different strains of pea, such as purple- flowered plants crossed with white-flowered plants. By 1863, he had made 287 different crosses between different strains of pea, and had grown and studied about 28,000 plants.
   2. In 1865, Mendel presented his results in this paper, published in a local natural history society's journal. No one took much notice of it at the time. . .
   3. . . . Mendel later gave up science, became Abbot (head of the monastery) and spent the rest of his life governing the monastery, dying in 1884 at the age of 61. . .
   4. . . . and it wasn't until 1900 than anyone realized what an amazing thing Mendel had done.

II. Mendelian Genetics.

1. Inheritance of Traits
   1. Let's look at one of the many crosses that Mendel performed. He crossed a purple- flowered pea plant with a white-flowered pea plant. (Call this the parental generation, or P for short.)
      1. He didn't get light-purple flowers, or pink flowers. All of the offspring peas (the first filial generation, or F₁ for short) were just as purple as their purple parent. None were white.
      2. But when Mendel crossed two of the F₁ plants, to get the second filial or F₂ generation, white-flowered plants appeared in the F₂ generation along with purple-flowered plants. These white-flowered plants, if allowed to self-fertilize, would produce white flowers for as many generations as Mendel could raise.
      3. Conclusion: Somehow the characteristic of white flower color had been passed from the P generation to the F₂ generation. It had been present in the F₁ plants, but had somehow been covered up, or masked, by the purple color. White flower color never "masked" purple color.
      4. A trait like purple flower color, that can hide the existence of other traits, Mendel called dominant. A trait like white flower color, that can be masked by another trait and yet reappear later, Mendel called recessive.
      5. Inheritance isn't like mixing paint -- it involves passing on some sort of discrete "particles" -- which Mendel called "elements," and which we now identify as genes.
   2. So what? Animal and plant breeders already knew that you sometimes get "throwbacks" to an ancestral trait. But what made Mendel's work so great was his willingness to count the pea plants. . .
2. Statistical Study of Traits
   1. In the F₂ generation of the purple-white cross experiment, Mendel noted that both purple-flowered and white-flowered plants appeared. Counting the number of each revealed that there were about three times as many purple-flowered plants as white-flowered plants.
      1. One-third of the purple-flowered F₂ plants, when allowed to self- fertilize, always produced only purple-flowered offspring.
      2. The other two-thirds of the purple-flowered plants, allowed to self- fertilize, produced both purple-flowered and white-flowered offspring.
      3. Thus, the F₂ generation broke down as follows: 25% true-breeding purple, 25% true-breeding white, and 50% hybrid purple.
   2. These relationships held up for the F₂ generation of every cross Mendel made -- yellow and green peas, yellow and green pods, tall and dwarf plants, etc.
   3. The explanation that Mendel offered was this:
      1. Every organism had its traits -- flower color, height, seed color, etc. -- determined by "particles" inside every cell, which we now call genes.
      2. There may be several different versions of each gene -- called alleles of that gene. (Thus we can talk about the purple-flower allele, the white-flower allele, etc.)
      3. Some alleles of one gene are dominant over other alleles of that gene -- meaning that their presence masks the presence of the other, recessive alleles.
      4. Each organism has, not one, but two genes for each trait.
      5. We can write alleles as single letters, with dominant ones as capitals and recessives in lower-case. For instance, we can use P for the dominant purple allele and p for the recessive white allele. A plant might be PP, Pp, or pp. Both PP and Pp plants will have purple flowers, and pp plants will have white flowers.
      6. Each parent normally passes on only one of its pair of genes to each offspring. A true-breeding plant with PP alleles can only pass P alleles on to its offspring. A pp plant can only pass on p alleles to its offspring. However, a hybrid plant with Pp alleles can pass on either P or p alleles to any one of its offspring.