Dunn, Leslie Clarence

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(b. Buffalo, New York, 2 November 1893; d. New York City, 19 March 1974)


Dunn’s parents, Clarence Leslie Dunn and Mary Eliza Booth Dunn, both from farming families, were high school graduates. Leslie went to Dartmouth College on a scholarship, and there his interest in zoology was awakened by John H Gerould, an inspiring teacher with a strong interest in the new science of genetics. Dunn graduated in 1915 (Phi Beta Kappa) and then went to Harvard University where he worked under William Ernest Castle. For his D.Sc. dissertation (1920) Dunn made linkage studies on certain genes in mice and rats.

Dunn served as an officer in the U.S. Army in France during World War I. He returned to Harvard in 1919. In May 1918 he had married Louise Porter. They had two sons, the younger of whom, Stephen, in spite of a spastic disability, became a collaborator with his father in anthropological and genetic studies of the Jewish community in Rome in the 1950’s.

Dunn’s first appointment, in 1920, was to the post of poultry geneticist at the Agricultural Experiment Station at Storrs. Connecticut. In the ensuing eight years he did pioneering work on the genetics of chickens, In scientific insight, versatility, and care in experimental design and execution, that work has probably never been surpassed. Dunn determined the relation between egg weight and hatchability, and the susceptibility of egg size to natural as well as artificial selection. He also explored the loss of vigor resulting from inbreeding and the restoration of vigor by outcrosses between different breeds. Much of this work was done in collaboration with a close friend and colleague, Walter Landauer.

While at Storrs. Dunn entered into collaboration with the plant geneticist Edmund Ware Sinnott to write an introductory textbook, Principles of Genetics (1925). Through two subsequent editions (1932, 1939) and two further revisions (1950. 1958), of which Theodosius Dobzhansky became a leading coauthor, this text remained one of the foremost in its field. Scarcely any American geneticist active in the five decades after its first publication did not owe it a great debt, either as student or as teacher. Masterly exposition and an extensive use of carefully designed problems for students to solve characterized all revisions.

In 1928 Dunn was appointed professor of zoology at Columbia University, to succeed Thomas Hunt Morgan. The facilities at Columbia were not suitable for work with poultry, so Dunn decided to continue some work with Drosophila, in the Columbia tradition, and to resume his studies of the genetics of mice, which he had never abandoned entirely. He was fortunate in recruiting excellent graduate students to work with him, and these were joined by visiting investigators and postdoctoral associates, especially Salome Gluecksohn-Schoenheimer (later Waelsch), who was trained under Hans Spemann in embryology and who from 1938 to 1954 collaborated with Dunn in many studies of abnormal mouse development caused by mutant genes or specific genotypes. Her place was later filled by Dorothea Bennett, who continued with Dunn to analyze the extraordinary genetic variability at the T = t locus (short tail or tailless phenotypes) arising by repeated mutations and widespread in mouse populations. The evolutionary forces that could lead to the spread of such lethal or near-lethal genes in populations fascinated Dunn. This prolonged investigation was probably his greatest original work in genetics.

The T mouse carries a dominant gene that produces a short tail. A recessive t gene, when homozygous (tt), produces taillessness. The compound, surprisingly, is quite viable and manifests only the shorttail characteristic. When Tt mice are bred together, neither homozygous TT nor tt offspring are born; only Tt mice are produced. As the Tt cross breeds true, T and t appear to be alleles, located in the same chromosome pair at the same locus.

Dunn suspected that the Tt mice bred true because both homozygous types (TT and tt) are lethal and do not recombine by crossing over. Hence he looked for deaths during embryonic life in the litters produced from TT by tt matings, and he did find two classes of lethal embryos, so that half of all the embryos were dying. Such a genetic situation had previously been found only in the fruitfly Drosophila, by H. J. Muller. But why, if T and t are alleles. do they show no interaction in effect?

Next a totally unexpected result turned up. In crosses of Tt or tt males by normal females, there was a great excess of offspring receiving the t gene from its male parent, although when females of those types were crossed to normal males, equal transmission of T and t occurred. Dunn recognized that he had found a situation in which a deleterious mutant gene could maintain itself in a population, by means of its superior ability to be transmitted through the spermatozoa, He therefore began a long search of many laboratory and wild populations of mice to find whether he could detect other cases of t alleles with this startling evolutionary propensity to counteract natural selection. Many were found. Thus, some detrimental genes can survive in populations and contribute materially to the “genetic load” burdening the population.

Dunn was later able to show, by means of crosses to new “marker” genes closely linked to T (about 6 units), that t mutant genes completely suppress recombination between T and the markers. In actuality, then the Tt system is not a single locus. but a chromosome segment carrying genes of related function. This was the first discovery of such clusters of genes with similar or associated functions in a mammal. It was a situation of both evolutionary and teratological importance, for such regions, or segments, of related function are now known to be widespread among species, including the human one. They demonstrate, on the one hand, how genes of related function may, by a process of tandem duplication, become multiplied and thereafter diverge somewhat in function. On the other hand, they reveal the similar, if not quite identical, nature of closely linked genes that cause similar developmental abnormalities. In both these respects, Dunn’s studies of the Tt genes of mice illuminate current studies of the human genome.

From the early 1920’s Dunn actively investigated the genetics of human races and populations. Race mixture in Hawaii, the genetic isolate of the Jews in Rome, and blood group research were interests that led him to establish the Institute for the Study of Human Variation (1952–1958), Dunns contributions to human genetics were recognized by his election in 1961 to the presidency of the American Society of Human Genetics.

During the 1930’s the rise of Nazi race policies and anti-Semitism in Germany greatly distressed Dunn. He labored intensively to aid scientist refugees arriving in America. During World War II his social sympathies led him to participate in the AmcricanSoviet Friendship Council, which he helped to found, and the American-Soviet Science Society. Lysenkoism in Russia altered his feelings toward the Soviet Union, but his associations brought him some political persecution in the 1950’s. In the scientific community Dunn always stood strongly for academic freedom and scientific integrity, and was a leader upon whom many young scientists looked with reverence.

His friendship with Milislav Demerec led Dunn to make many influential suggestions in the organization of the great Cold Spring Harbor symposia on genetics and evolution in the 1940’s and 1950’s. He was renowned as an editor; he was the first editor of Genetics (1935–1940), and the editor of American Naturalist (1951–1960), of Genetics in the Twentieth Century (1951) and of Race and Biology prepared for UNESCO (1951; 3rd ed. 1970). His popular books Heredity and Variation (1932), Heredity, Race and Society (written with Theodosius Dobzhansky, 1946; 4th ed. 1972), Biology and Race (1951), and Heredity and Evolution in Human Populations(1959), together with many articles in encyclopedias and other compendia, exerted a wide influence. Dunn possessed a strong historical sense, which bore fruit in a final book, A Short History of Genetics 1864–1939 (1965), as well as in the impetus he gave to the formation of a rich archive of geneticists’ documentary materials at the American Philosophical Society,

Dunn received a great many honors, including an honorary Sc.D degree from Dartmouth College, and election to the National Academy of Sciences, the American Philosophical Society, the Accademia Pataviana. the Norwegian Academy of Sciences, and the presidency of the Genetics Society of America (1932) and of the American Society of Naturalists (1960).


I. Original Works. A full bibliography is appended to the memoir of Dunn by Theodosius Dobzhansky. in Biographical Memoirs, National Academy of Sciences, 49 (1978), 79–104. The L. C. Dunn papers, fully indexed, are preserved in the library of the American Philosophical Society.

II. Secondary Literature. See Joseph, S.Fruton, A Bio-bibliography for the History of the Biochemical Sciences Since 1800 (Philadelphia, 1982) for biographical listings. See also Beutley Glass, A Guide to the Genetics Collections of the American Philosophical Society (Philadelphia. 1988). Dorothea Bennett, “L. C. Dunn and His Contribution to T-Locus Genetics,” in Annual Review of Genetics, 11 (1977), 1–12, is an exceptionally rich and full account of that major scientific work,

Bentley Glass