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In 1851 Austrian monk Gregor Mendel was sent by his monastery to the University of Vienna to study mathematics and science. Upon his return in 1854, be began teaching science, and several years later he began his now-famous series of hybridization experiments with garden peas in the monastery's small garden. There Mendel cross-pollinated plants of different sizes and shapes as well as plants that produced different colored flowers or peas. Analyzing each generation of new plants, he observed that some characteristics remained constant (or dominant ) in every generation, while others remained hidden (or recessive ), appearing only in later generations. Mendel called the units of inheritance factors and proved that two such factors, one from each parent, exist for each trait. A dominant trait requires only one factor for it to appear, but a recessive trait requires both. We now call the units of inheritance genes, and we know that each gene, which is composed of a DNA sequence, occupies a specific place on the chromosome. Although Mendel's rules of inheritance were published in 1865 in his article “Versuche über Pflanzenhybriden” (“Experiments with Plant Hybrids”), his work was ignored until 1900, when it was rediscovered by the Dutch botanist Hugo de Vries (1878–1918).