Mendel’s Laws

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Mendel’s Laws, principles of hereditary transmission of physical characteristics. They were formulated in 1865 by the Augustinian monk Gregor Johann Mendel. Experimenting with seven contrasting characteristics of pure-breeding garden peas, Mendel discovered that by crossing tall and dwarf parents, for example, he got hybrid offspring that resembled the tall parent rather than being a medium-height blend. To explain this he conceived of hereditary units, now called genes, which often expressed dominant or recessive characteristics. Formulating his first principle (the law of segregation), Mendel stated that genes normally occur in pairs in the ordinary body cells, but segregate in the formation of sex cells (eggs or sperm), each member of the pair becoming part of the separate sex cell. When egg and sperm unite, forming a gene pair, the dominant gene (tallness) masks the recessive gene (shortness).

To corroborate the existence of such hereditary units, Mendel went on to interbreed the first generation of hybrid tall peas and found that the second generation turned out in a ratio of three tall to each short offspring. He then correctly conceived that the genes paired into AA, Aa, and aa (“A” representing dominant and “a” representing recessive). Continuing the breeding experiments, he found that the self-pollinated AA bred true to produce pure tall plants, that the aa plant produced pure dwarf plants, and that the Aa, or hybrid, tall plants produced the same three-to-one ratio of offspring. From this Mendel could see that hereditary units did not blend, as his predecessors believed, but remained unchanged from one generation to another. He thus formulated his second principle (the law of independent assortment), in which the expression of a gene for any single characteristic is usually not influenced by the expression of another characteristic. Mendel's laws became the theoretical basis for modern genetics and heredity.

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