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Sturtevant had a prodigious memory and truly encyclopedic interests. He had a natural bent for mathematics but little formal training in it. He especially enjoyed, and was expert at solving, all kinds of puzzles, especially those involving geometrical situations. For him scientific research was an exciting and rewarding activity not unlike puzzle-solving. a common theme of his investigations was an effort to analyze and explain exceptions to established principles. Sturtevant knew how to design and execute simple, elegant experiments, describing the results in concise, lucid prose. He set high standards for his own research and expected others to do the same. Sturtevant's discoveries of the principle of gene mapping, of the first reparable gene defect, of the principle underlying fate mapping, of the phenomena of unequal crossing-over, and of position effect were perhaps his greatest scientific achievements. Mendel had found that all of the hereditary factors with which he worked assorted independently of one another at the time of gamete formation. Exceptions to this second Mendelian law began to accumulate in 1900–1909. Morgan was the first to provide a satisfactory explanation for such exceptions in terms of a hypothesis, which assumes that genes tending to remain together in passing from one generation to the next must be located in the same chromosome. He further postulated that the extent to which such linked genes recombine at meiosis is a relative measure of their physical distance. Sturtevant introduced the concept that the frequency of crossing-over between two genes furnishes an index of their distance on a linear genetic map. He proposed that 1 percent of crossing-over be taken as equal to one map unit. He then reasoned that if the distance between two genes, A and B, is equal to x map units and the distance between B and a third gene, C, is equal to y map units, then the distance between A and C will be x y if B is the middle gene; x - y if C is the middle gene, and y - x if A is the middle gene. The germ of this idea occurred to Sturtevant in conversation with Morgan. In his History of Genetics, Sturtevant recorded that he “went home, and spent most of the night (to the neglect of my undergraduate homework) in producing the first chromosome map, including the linked genes, y, w, v, m, and r, in that order, and approximately the relative spacing, as they still appear on the standard maps“ (p. 47). Sturtevant devised a crucial test of the principles of mapping genes by constructing crosses in which all three genes were segregating simultaneously. In the progeny of such “three-factor“ crosses, Sturtevant discovered that double crossing-over can occur and that its frequency is equal to, or less than, the product of the two single crossing-over frequencies. Conversely, the frequency of double crossing-over can be used to deduce the order of the three genes. Sturtevant showed that the order obtained from two-factor crosses was fully confirmed and that the three-factor crosses provided a more powerful method of ordering and mapping genes than did two-factor crosses. He published these findings in 1913. His principles and methods of chromosome mapping have enabled geneticists to map the chromosomes of a wide variety of higher organisms, including man. #mendel #Morgan #Drosophila #geneMapping #chromosomeMapping #genetics #Sturtevant
