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American biologist (1891–1970)

Alfred Henry Sturtevant was born in Jacksonville, Illinois, United States on November 21, 1891, the youngest of Alfred Henry and Harriet Sturtevant's six children. His grandfather Julian Monson Sturtevant, a Yale University graduate, was a founding professor and second president of Illinois College, where his father taught mathematics. When Sturtevant was seven years old, his father quit his teaching job and moved the family to Alabama to pursue farming. Sturtevant attended a one room schoolhouse until entering high school in Mobile. In 1908, he enrolled at Columbia University. During this time, he lived with his older brother Edgar, a linguist, who taught nearby. Edgar taught Alfred about scholarship and research. In 1914, Sturtevant completed his doctoral work under Morgan as well. He became the leader of a new genetics research group at Caltech, whose members included George W. Beadle, Theodosius Dobzhansky, Sterling Emerson, and Jack Schultz. He was elected to the United States National Academy of Sciences in 1930 and the American Philosophical Society in 1936. He was elected a Fellow of the American Academy of Arts and Sciences in 1949. Sturtevant was awarded the John J. Carty Award from the National Academy of Sciences in 1965 On February 13, 1968, he received the 1967 National Medal of Science "For a long and distinguished career in genetics during which he discovered and interpreted a number of important genetic phenomena in Drosophila and other organisms." In this same time frame, Sturtevant was an outspoken opponent of eugenics and was interested in the effects of the atomic bomb on human populations, due to his previous research on lethal genes. He warned the public of possible harmful genetic effects of nuclear fallout despite supposedly low levels of ionizing radiation.

In 1865, Gregor Mendel published a paper entitled “Experiments in Plant Hybridization,” in which he proposed the principles of heredity. This paper introduced the concept of dominant and recessive genes to explain how a characteristic can be repressed in one generation but appear in the next generation. Mendel also assumed that all hereditary factors worked independently of one another, which he explained in his law of independent assortment. Mendel's paper did not achieve much acclaim and was largely forgotten until 1900. During this time frame, Hugo de Vries put forth a theory that persistent hereditary units are passed through generations and that each “unit” deals with a specific characteristic and the units can combine in different ways in the offspring.

Sturtevant's most notable discoveries include the principle of genetic mapping, chromosomal inversion, the first observation of a single gene mutation affecting behavior, the first reparable gene defect, the principle underlying fate mapping, the phenomena of unequal crossing over, and position effect. His main contributions to science include his analysis of genetic “linkage groups,” which became a classical method of chromosome mapping that we still use today. He also showed that the gene for any specific trait was in a fixed location (locus). In his work between 1915 and 1928, Sturtevant determined that genes of Drosophila are arranged in linear order. In 1920, he published a set of three papers under the title “Genetic Studies on Drosophila simulans,” which “proved that two closely related species had newly recurring mutations that were allelic and thus probably identical”. His work also helped to determine the chromosomal basis of sex determination and development and described the importance of chromosomal crossing over or recombination in genetic linkage of traits. One of Sturtevant's principal contributions was his introduction of the concept that the frequency of crossing over between two genes could help determine their proximity on a linear genetic map. His experiments determined that the frequency of double crossing over can be used to deduce gene order. He demonstrated this concept by constructing crosses of three segregating genes, called "three factor crosses". He found that using three genes as opposed to two provided most accurate information about gene order on chromosome. With this system, Sturtevant discovered that double crossing over occurs at frequency of equal to or less than the product of two single crossing over frequencies. He also demonstrated the possibility of "unequal crossing over" and speculated that it was possibly a main force of evolution.