At the back of his copy of Gärtner's Versuche und Beobachtungen über die Bastarderzeugung im Pflanzenreich (1849) ("Experiments and observations of hybridization in the plant kingdom"), Mendel made a series of annotations about the characteristics of different varieties of plants derived from Gärtner's experiments with peas. Here Mendel seems to be at work on the preparatory phase of his experiments, as he singles out constant characters (i.e. that do not vary between generations) that can be observed in the varieties of Pisum.
The illustrated Album (1876) by Ernst Benary, a renowned hybridist, who also knew about Mendel's experiments, and whose namesakes' company's seeds Mendel ordered for the Abbey, devotes two pages to peas, and one of these effectively illustrates three of the traits selected by Mendel for his experiments: coloured and white flowers, yellow and green pods, straight (rigid walls) and beaded pods (i.e. without skin).
 Mendel used thirty-four varieties of Pisum sativum, (sub-species and convarieties of the garden pea). This plant had been recommended by other biologists, such as Kölreuter in Germany, Seton and Goss in England, because of its significantly large flowers and wide range of variations, such as length and colour of the stem, size and form of the leaves, position/colour of the flowers, length of the flower stalk, etc. Furthermore the Pisum is an "inbreeder" plant (it is self-fertile) and a "true breeder" (the off-springs will resemble their parents, unless it is artificially fertilized, or cross-bred).
The original coloured drawings of the plans for the greenhouse built in the Abbey's garden by Abbot Napp [i] show the shape and size of the greenhouse where Mendel carried out his experiments with peas.
Absolutely crucial to Mendel's experiments was that the characters he selected bred true (the plants were true-breeders), and he spent two seasons verifying this. Carefully controlling his experiments he reflected that some of the characters did not permit "sharp and certain separations since the differences of the ‘more or less' nature'" are often difficult to define. He therefore selected seven characters which "stand out clearly and definitely in the plants". These were:
  form of the ripe seeds (round or roundish, or angular and wrinkled)
  colour of the seed (pale yellow, bright yellow and orange-coloured, or green)
  colour of the seed coat (either white or grey, grey brown, leather brown, with or without violet spotting)
  form of the ripe pods (simply inflated or deeply constricted and more or less wrinkled)
  colour of the unripe pod (light to dark green or vividly yellow)
  position of the flowers (axial, i.e. distributed along the main stem, or terminal, this is bunched at the top of the stem)
  length of the stem (the long axis of six to seven feet, or a short one of 3/4 to 1 feet)
 The crucial importance of Mendel's experimentation with peas lies in these fundamental facts: characters or traits from parents pass as unmodified "units", individual "Mendelian" factors (which we now call "genes") to successive generations according to set ratios. Individuals possess two sets of factors: one of each received from either parent. It makes no difference if any one character is inherited from male or female: they both contribute in the same way. Furthermore these factors are sometimes expressed and sometimes concealed but never lost. Generally (but not always) each "unit" is passed on independently from all other "units". For example, a pea may develop seeds that are either round and yellow, or wrinkled and yellow, or round and green, or wrinkled and green. The expression of these characteristics within generations of plants was observed to be variable. Certain characters were manifested much more frequently than others (even when they had been present in equal numbers in previous generations). It appears that certain characters were not immediately expressed (recessive or hidden) in preference to others (dominant). When two plants were crossed, the ratio of characters in the resulting generations could be expected to have been 1:1. However, what Mendel observed and was able to repeat, was that one character (dominant) appeared three times as frequently as another (recessive). The average ratio observed through carefully controlled experiments was 3:1.
Mendel's experiments were systematically recorded over a period of seven years, from 1856 to 1863 and he read a paper on the ensuing results at a session of the Society of Natural Sciences in 1865 in Brno. This paper was published the following year in the extracts of the Society. On show are the relevant pages from both the manuscript and the printed version of Mendel's Versuche, containing his epoch-making results. [i]
 Mendel also experimented with varieties of the Hieracium plant (hawkweed) [i]. The selected characters in the hybrids proved to be highly unstable and the results did not match those obtained with Pisum sativum. In Notizblatt II, [i] one of the two of fragments of Mendel's note sheets, he compares the results of his experiments with Pisum and Hieracium and those obtained with Salix (willow). The interpretation of Mendel's Notizblatt I [i] is much debated. The notes record a re-examination of numerical data in plant hybridisation which Mendel probably conducted many years after the publication of the "Versuche". It is difficult to follow precisely Mendel's thinking in these later notes. However we learn that Mendel's botanical experiments carried on after the publication of his original paper and that he continued to be preoccupied with solving the enigma of generation.
Between 1866-1873 Mendel corresponded with Carl Nägeli (1817-91), Professor of Botany at the University of Munich and an authority in plant hybrids. Nägeli was convinced that hybrids were generally unstable and he could not agree with Mendel's theory that the characters passed onto hybrids from their parents were constant. The period of Mendel's correspondence with Nägeli [i] coincides with the experiments with the Hieracium which disappointingly seemed to prove Nägeli right. Mendel's observed what he called "a peculiar behaviour of the hybrids" which he was unable to explain - i.e. that the Hieracium exhibited both sexual and a-sexual reproduction (a phenomenon known as apomixis).
The importance of Mendel's work was recognised only thirty years after the publication of his seminal paper, when Hugo de Vries in 1900 in Holland, William Bateson in 1902 in Great Britain, Franz Correns in 1900 in Germany, and Erich Tschermak in 1901 in Austria were all to acknowledge Mendel's legacy, and hail him as the true father of classical genetics.
Peas and Hawkweed
Mendel's paper and notes
Mendel- Nägeli letters
Glass house drawings
  From Mendel to the Human Genome Project
1856
Gregor Mendel begins experiments cross-breeding the garden pea.
1859
Charles Darwin publishes On the Origin of Species.
1865
Mendel presents his paper on the results and his interpretation of his experiments, at the monthly meetings, 8 February and 8 March, of the Naturforschenden Vereins in Brno, the Natural Science Society.
1866
Mendel publishes Versuche über Pflanzen-hybriden in the Society's journal. He sends out offprints but these are ignored.
1871
Friedrich Miescher reports the discovery of nuclein from isolated cell nuclei; though nuclein is now understood to be a mixture of nucleic acids (DNA and RNA) and protein, this is the first work with nucleic acids.
1887
August Weisman, under the microscope, disentangles the dance of the chromosomes in meiosis (formation of the germ cells). He points out that each chromosome must carry a great number of determinants of hereditary characters, and speculates about the minute size of them.
1900
Carl Correns and Hugo de Vries, working independently, rediscover Mendel's rules and then his paper; Erich von Tschermak plays a minor role. William Bateson publicises Mendel's work to the Royal Horticultural Society of London and soon translates his paper.
1902
Walter Sutton promotes the chromosome theory of inheritance, namely, that chromosomes can be seen to behave just like the Mendelian elements (soon to be called genes) and that each chromosome must carry many of them; Theodor Boveri makes a similar observation.
1902-1909
Bateson coins the terms genetics, allelomorph, (now shortened to allele), homozygote, heterozygote and others.
1909
Archibald Garrod originates the subspecialty of biochemical genetics by demonstrating that certain human diseases are inborn errors of metabolism, inherited as Mendelian recessive characters.
1910
Thomas Hunt Morgan finds a mutant eye-colour in the fruitfly, Drosophila, and discovers sex linkage. He proposes that the genes located on the same chromosome are linked together and can recombine by exchange of chromosome segments, called crossing over.
1913
Alfred Henry Sturtevant draws the first genetic map, using cross-over frequencies between six sex-linked Drosophila genes to show their relative locations on the X chromosome.
1927
Hermann Muller demonstrates that x rays can induce genetic mutations in Drosophila.
1931
Harriet Creighton and Barbara McClintock, and Curt Stern independently, find the first direct proof, in cells under the microscope, that crossing-over takes place.
1941
George Beadle and Edward Tatum, working with the biochemical genetics of the mold Neurospora, propose the "one gene-one enzyme" theory.
1940s
Max Delbrück and Salvador Luria, and Jacques Monod independently, demonstrate that bacteria have genes.
1944
Oswald Avery, Colin MacLeod, and Maclyn McCarty publish strong evidence that DNA is the hereditary material.
1949
Frederick Sanger, working with the insulin molecule, publishes the first evidence that the sequence of amino acids in a protein chain is unique to that protein.
1949
Erwin Chargaff publishes evidence that the proportions of the four kinds of nucleotides, the components that make up a strand of DNA, are the same in all cells of a given creature but vary greatly from one species to another. DNA is set free to carry genetic information.
1953
James Watson and Francis Crick elucidate the three-dimensional molecular structure of DNA, the double helix. They relied partly on unpublished x-ray crystallographic data obtained by Rosalind Franklin and by Maurice Wilkins.
1957-'58
Crick proposes the Central Dogma of molecular biology, which states that genetic information, meaning specific sequences, can move among nucleic acids and into protein, "but that once information has passed into protein it cannot get out again".
1958
Matthew Meselson and Franklin Stahl demonstrate that DNA replicates semi-con-servatively, as the Watson-Crick structure requires.
1950s
François Jacob and Jacques Monod, in a long series of experiments, establish the existence of control functions located on the chromosome which turn the expression of genes on or off.
1961
Jacob, Crick, Sydney Brenner, and others work out the general scheme of the transcription of the information in DNA into messenger RNA and the translation of mRNA into protein. Brenner, Jacob, and Meselson find mRNA in bacterial cells.
1961
Crick and colleagues demonstrate that the genetic information is carried in three-nucleotide sets, called codons, of which there are 64, each coding for one of the 20 amino acids of protein chains; three codons turn out to be stop signals.
1961
Marshall Nirenberg and Heinrich Matthaei get the first and second codons to be identified.
1961-'67
Nirenberg, Severo Ochoa, H. Gobind Khorana, and others, in a furious race, determine the rest of the genetic code.
1970s
Paul Berg, Stanley N. Cohen, and others develop methods for cutting DNA up and recombining the fragments in novel sequences: recombinant DNA, or genetic engineering, is launched.
1977
Sanger invents a remarkable method for sequencing DNA; Walter Gilbert and Allan Maxam independently devise another.
1983
For the first time, the gene defect that causes a human disorder-Huntington's disease-is located exactly on a chromosome and so isolated for study.
1995
First bacterial genome sequenced (Haemophilus influenzae).
1995
Genomes sequenced of bacteria Mycoplasma genitalium and Escherichia coli, yeast
2000
(Saccharomyces cerevisiae), roundworm (Caenorhabditis elegans), fruitfly (Drosophila melanogaster) and mustard cress (Arabidopsis thaliana).
1999
First human chromosome sequenced (chromosome 22).
2002
The mouse genome sequenced.
2003
First full draft of the human-genome sequence completed.
The Mendel Museum wishes to thank Horace Freeland Judson for preparing this overview. Thanks are also due to Jonathan Hodgkin and Jirina Relichova.