Gregor Johann Mendel(redirected from Mendel second law)
Also found in: Dictionary, Thesaurus, Medical.
Mendel, Gregor Johann(grā`gôr yō`hän mĕn`dəl), 1822–84, Austrian monk noted for his experimental work on heredityheredity,
transmission from generation to generation through the process of reproduction in plants and animals of factors which cause the offspring to resemble their parents. That like begets like has been a maxim since ancient times.
..... Click the link for more information. . 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anticipated Oscar Hertwig's discovery that fertilization of an egg involved only one male sex cell, these findings went unpublished.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 geneticsgenetics,
scientific study of the mechanism of heredity. While Gregor Mendel first presented his findings on the statistical laws governing the transmission of certain traits from generation to generation in 1856, it was not until the discovery and detailed study of the
..... Click the link for more information. 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 genesgene,
the structural unit of inheritance in living organisms. A gene is, in essence, a segment of DNA that has a particular purpose, i.e., that codes for (contains the chemical information necessary for the creation of) a specific enzyme or other protein.
..... Click the link for more information. , 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 meiosismeiosis
, process of nuclear division in a living cell by which the number of chromosomes is reduced to half the original number. Meiosis occurs only in the process of gametogenesis, i.e., when the gametes, or sex cells (ovum and sperm), are being formed.
..... Click the link for more information. ) 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).
Mendel, Gregor Johann
Born July 22, 1822, in Heinzendorf, Austria-Hungary (present-day Hinčice, Czechoslovakia); died Jan. 6, 1884, in Briinn, Austria-Hungary (present-day Brno). Discoverer of Mendelism, the theory of heredity. Son of a peasant.
Mendel became a monk in the Augustinian monastery in Briinn in 1843 (and an abbot in 1868) because of financial difficulties after completing philosophy courses at the university in Olmütz. In 1849 he taught natural history and physics in a high school. From 1851 to 1853 he took courses in physics, botany, paleontology, and analytical chemistry at the University of Vienna.
Between 1856 and 1863, Mendel performed extensive experiments with the hybridization of 22 varieties of pea plant. He reported the results of these experiments to the Briinn Natural Science Society in 1865; his paper was published in the proceedings of the society in 1866. His quantitative recording of all types of hybrids obtained, as well as his variational-statistical approach, characteristic of the entire body of his work, made Mendel the first to substantiate and formulate the principles of the random segregation and recombination of hereditary factors. These principles became the basis of his theory of heredity, known as Mendel’s laws.
Mendel tried to confirm his principles with other plants, including hawkweed. The choice of hawkweed was unfortunate, since the results contradicted the data obtained for peas. (It was later discovered that hawkweed often reproduces without fertilization, so that hybrids cannot be obtained.) He also concerned himself with apiculture, meteorology, and horticulture (breeding a new variety of fuchsia and doing grafts and crosses of fruit trees), and he crossed gray and white mice.
Mendel’s discoveries failed to win him recognition during his lifetime, although the results were known to a number of the leading botanists of his day. His misunderstood and forgotten work first attracted widespread attention in 1900, when H. De Vries, C. Correns, and E. von Tschermak became convinced almost simultaneously of the soundness of his conclusions on the basis of their own experiments. The international scientific community celebrated the centenary of Mendel’s discoveries in 1965.
WORKSOpyty nad rastiteVnymi gibridami (s biografich. ocherkom). Moscow, 1965.
[Soch.] In G. Mendel, C. Naudin, and A. Sagiret, Izbrannye raboty. Moscow, 1968.
REFERENCESFilipchenko, lu. A. Frensis GaVton i Gregor Mendel’. Moscow, 1925.
Timiriazev, K. “Mendel’.”In Entsiklopedicheskii slovar’ Granat, llth ed., vol. 28. Moscow [no date].
Gaisinovich, A. E. Zarozhdenie genetiki. Moscow, 1967.
Orel, V. “Kak rodilas’ teoriia Mendelia.” Priroda, 1972, no. 5.
Iltis, H. Gregor Johann Mendel: Leben, Werk und Wirkung. Berlin, 1924.
Gregor Johann Mendel, 1822-1884: Texte und Quellen zu seinem Werken und Leben. Leipzig, 1965. (Compiled with commentary by J. Kříženecký.)
Jakubíček, M., and J. Kubíček. Bibliographia Mendeliana. Brno, 1965.
Jakubíček, M., and J. Kubíček. Bibliographia Mendeliana. Supple. 1965-1969. Brno, 1970.
Folia Mendeliana. (Annually, since 1966.)
Gustafsson, A. “The Life of Gregor Johann Mendel—Tragic or Not?” Hereditas, 1969, nos. 1-2.