Mendel, Gregor Johann
Gregor Johann Mendel (grā´gôr yō´hän mĕn´dəl), 1822–84, Austrian monk noted for his experimental work on heredity. He entered the Augustinian monastery in Brno in 1843, taught at a local secondary school, and carried out independent scientific investigations on garden peas and other plants until his election as prelate in 1868. Failing eyesight and his duties as prelate somewhat curtailed his researches; although he anticipated Oscar Hertwig's discovery that fertilization of an egg involved only one male sex cell, these findings went unpublished.
Mendel was the first to fashion, by means of a controlled pollination technique and careful statistical analysis of his results, a clear, analytic picture of heredity. His account of the experiments and his conclusions, published in 1866 (tr. Experiments in Plant Hybridization, 1926), were ignored during his lifetime. Rediscovered by three separate investigators (Correns, de Vries, and Tschermak) in 1900, Mendel's conclusions have become the basic tenets of genetics and a notable influence in plant and animal breeding.
Mendelism is the system of heredity formulated from Mendel's conclusions. Briefly summarized, as we understand it today by means of the science of genetics, the Mendelian system states that an inherited characteristic is determined by the combination of a pair of hereditary units, or genes, one from each of the parental reproductive cells, or gametes. In the body cells each pair of genes determines a particular hereditary characteristic (e.g., in the pea plant, a pair determining tallness or dwarfness).
Mendel's First Law
The law of segregation (Mendel's first law) states that in the process of the formation of the gametes (see meiosis) the pairs separate, one going to each gamete, and that each gene remains completely uninfluenced by the other. Mendel found that when a pure strain of peas bearing one form of a gene (that is, a strain in which both members of the gene pair being studied are the same), inbred for many generations, was crossed with a pure strain carrying an alternative form of the gene, one of these forms consistently prevailed over the other in determining the visible characteristics of the offspring; he therefore termed the two forms dominant and recessive, and called the phenomenon itself the law of dominance. Given A as the dominant factor and a as the recessive, the offspring of the purebred strains having genes of the form AA and aa are hybrids, individuals each being Aa. When the hybrids are crossed, the offspring exhibit the characteristic in question in a ratio of three dominant to one recessive; i.e., the four possible combinations of the genes in Aa and Aa are AA,aA,Aa, and aa. By the same rule, when a hybrid is crossed with a purebred recessive (Aa with aa) the ratio is one to one. Breeders often use these ratios to trace the hybrid or purebred nature of the parent stock.
Mendel's Second Law
The law of independent assortment (Mendel's second law) states that characteristics are inherited independently of each other; e.g., the dominant trait of yellow seed color in pea plants can appear in combination with either the dominant trait of plant tallness or the recessive trait of dwarfness. This law has been modified by the discovery of linkage in genetics.
See biography of Mendel by V. Ore (1984); see also R. C. Olby, The Origins of Mendelism (2d ed. 1985).