A. H. Sturtevant

Alfred Henry Sturtevant (1891-1970) was a geneticist and National Medal of Science winner whose principles of gene mapping greatly affected the field of genetics.

A. H. Sturtevant, an influential geneticist and winner of the National Medal of Science in 1968, is best known for his demonstrations of the principles of gene mapping. This discovery had a profound effect on the field of genetics and led to projects to map both animal and human chromosomes. He is the unacknowledged father of the Human Genome Project, which is attempting to map all of man's 100, 000 chromosomes by the year 2000. Sturtevant's later work in the field of genetics led to discovery of the first reparable gene defect as well as the position effect, which showed that the effect of a gene is dependent on its position relative to other genes. He was a member of Columbia University's "Drosophila Group, " whose studies of the genetics of fruit flies advanced new theories of genetics and evolution.

Alfred Henry Sturtevant, the youngest of six children, was born in Jacksonville, Illinois, on November 21, 1891, to Alfred and Harriet (Morse) Sturtevant. Five of his early ancestors had come to America aboard the Mayflower. Julian M. Sturtevant, his grandfather, a Yale Divinity School graduate, was the founder and former president of Illinois College. Sturtevant's father taught at Illinois College briefly but later chose farming as a profession. When Alfred Sturtevant was seven, his family moved to a farm in southern Alabama. He attended high school in Mobile, which was 14 miles from his home and accessible only by train.

Sturtevant enrolled in Columbia University in New York City in 1908, boarding with his older brother, Edgar, who taught linguistics at Columbia's Barnard College. Edgar and his wife played a significant role in young Sturtevant's life. They sent him Columbia's entrance examination, pulled strings to get him a scholarship, and welcomed him into their home in Edgewater, New Jersey, for four years. Edgar was also responsible for steering his brother toward a career in the sciences. The young Sturtevant had discovered genetic theory at an early age and often drew pedigrees of his family and of his father's horses. Edgar encouraged him to write a paper on the subject of color heredity in horses and to submit the draft to Columbia University's Thomas Hunt Morgan, the future Nobel Laureate geneticist. The paper used the recently rediscovered theories of Gregor Mendel, the 19th-century Austrian monk and founder of genetics, to explain certain coat-color inheritance patterns in horses. Sturtevant somehow mastered this subject in spite of his color-blindness.


Student Work Leads to Major Genetic Breakthrough

As a result of his paper on horses, which was published in 1910, Sturtevant was given a desk in Morgan's famous "fly room, " a small laboratory dedicated to genetic research using Drosophila (fruit flies) as subjects. Fruit flies are ideal subjects for genetic research. They mature in ten days, are less than one-eighth inch long, can live by the hundreds in small vials, require nothing more substantial than yeast for food, and have only four pairs of chromosomes.

Morgan's early work focused on the phenomenon of "crossing-over" in the fruit fly. By 1910, he had already described the sex-limited inheritance of white eye. From this observation, he postulated the idea that genes were linked because they were carried by the same chromosome and that genes in close proximity to one another would be linked more frequently than those that were farther apart. Sometimes, dominant linked traits, such as eye color and wing size, became "unlinked" in offspring. Sturtevant studied the process of crossing-over of sex-linked traits, which are carried on the X chromosome. Female fruit flies have two X chromosomes. In addition to one X chromosome, males have a Y chromosome, which carries very few genes. Sturtevant correctly hypothesized that the exchange between X chromosomes probably occurred early on in the process of egg formation, when the paired chromosomes lie parallel to each other.

Morgan believed that the relative distance between genes could be measured if the crossing-over frequencies could be determined. From this lead, Sturtevant developed a practical method for determining this frequency rate. He began by studying six sex-linked traits and measured the occurrence of this related trait. The more frequently the traits occurred, Sturtevant reasoned, the closer the genes must be. He then calculated the percentages of crossing-over between the various traits. From these percentages, he determined the relative distance between the genes on the chromosome, the first instance of gene mapping. This major discovery, which Sturtevant published in 1913 at the age of 22, eventually enabled scientists to map human and animal genes. It is often considered to be the starting point of modern genetics.

In 1914, Sturtevant received his Ph.D. from Columbia and stayed on in Morgan's lab as an investigator for the Carnegie Institution of Washington, D.C. Along with C. B. Bridges, Hermann Joseph Muller, and Morgan, he formed part of an influential research team that made significant contributions to the fields of genetics and entomology. He later described the lab as highly democratic and occasionally argumentative, with ideas being heatedly debated. The 16 x 24-foot lab had no desks, no separate offices, one general telephone, and very few graduate assistants. Sturtevant thrived in this environment. He worked seven days a week, reserving his mornings for Drosophila research and his afternoons for reading the scientific literature and consulting with colleagues. He possessed a near photographic memory and wide-ranging interests. His only shortcoming as a researcher was his incessant pipe-smoking, which often left flakes of tobacco ash mixed in with the samples of fruit flies. In spite of this minor flaw, the fly-room group raised research standards and elevated research writing to an art form. They also perfected the practice of chromosome mapping, using Sturtevant's methods to develop a chromosome map of Drosophila, detailing the relative positions of fifty genes.

Sturtevant published a paper in 1914 that documented cases of double crossing-over, in which chromosomes that had already crossed-over broke with one another and recrossed again. His next major paper, published in 1915, concerned the sexual behavior of fruit flies and concentrated on six specific mutant genes that altered eye or body color, two factors that played important roles in sexual selection. He then showed that specific genes were responsible for selective intersexuality. In later years, he discovered a gene that caused an almost complete sex change in fruit flies, miraculously transforming females into near males. In subsequent years, researchers identified other sex genes in many animals, as well as in humans. These discoveries led to the development of the uniquely twentieth-century view of sex as a gene-controlled trait which is subject to variability.

During the 1920s, Sturtevant and Morgan examined the unstable bar-eye trait in Drosophila Drosophila. Most geneticists at that time believed that bar eye did not follow the rules of Mendelian heredity. In 1925, Sturtevant showed that bar eye involved a recombination of genes rather than a mutation and that the position of the gene on the chromosome had an effect on its action. This discovery, known as the position effect, contributed greatly to the understanding of the action of the gene.

In 1928, Morgan received an offer from the California Institute of Technology to develop a new Division of Biological Sciences. Sturtevant followed his mentor to California, where he became Caltech's first professor of genetics. The new genetics group set up shop in Caltech's Kerckhoff Laboratory. Sturtevant continued working with fruit flies and conducted genetic investigations of other animals and plants, including snails, rabbits, moths, rats, and the evening primrose, Oenothera.

In 1929, Sturtevant discovered a "sex ratio" gene that caused male flies to produce X sperm almost exclusively, instead of X and Y sperm. As a result, these flies' offspring were almost always females. In the early 1930s, giant chromosomes were discovered in the salivary glands of fruit flies. Under magnification, these chromosomes revealed cross patterns which were correlated to specific genes. The so-called "physical" map derived from these giant chromosomes did not exactly match Sturtevant's "relative" location maps. In the physical map, some of the genes tended to cluster toward one end of the chromosome and the distances between genes was not uniform. But the linear order of the genes on the chromosome matched Sturtevant's relative maps gene for gene. This discovery confirmed that Sturtevant had been correct in his assumptions about chromosomal linearity.

In 1932, Sturtevant took a sabbatical leave and spent the year in England and Germany as a visiting professor of the Carnegie Endowment for International Peace. When he returned to America, he collaborated with his Caltech colleague Theodosius Dobzhansky, a Russian-born geneticist, on a study of inversions in the third chromosome of Drosophila pseudoobscura. In the 1940s, Sturtevant studied all of the known gene mutations in Drosophila and their various effects on the development of the species. From 1947 to 1962, he served as the Thomas Hunt Morgan Professor of Biology at Caltech. His most significant scientific contribution during that time occurred in 1951, when he unveiled his chromosome map of the indescribably small fourth chromosome of the fruit fly, a genetic problem that had puzzled scientists for decades.

During the 1950s and 1960s, Sturtevant turned his attention to the iris and authored numerous papers on the subject of evolution. He became concerned with the potential dangers of genetics research and wrote several papers on the social significance of human genetics. In a 1954 speech to the Pacific Division of the American Association for the Advancement of Science, he described the possible genetic consequences of nuclear war and argued that the public should be made aware of these possible cataclysmic hazards before any further bomb testing was performed. One of his last published journal articles, written in 1956, described a mutation in fruit flies that, by itself, was harmless but which proved lethal in combination with another specific mutant gene.

Sturtevant married Phoebe Curtis Reed in 1923, and the couple honeymooned in Europe, touring England, Norway, Sweden, and Holland. The Sturtevants had three children. Sturtevant was named professor emeritus at Caltech in 1962. He spent the better part of the early 1960s writing his major work, A History of Genetics, which was published in 1965. In 1968, he received the prestigious National Medal of Science for his achievements in genetics. He died on April 5, 1970, at the age of 78.