Sturtevant's work and its importance
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. In 1913, he determined that genes were arranged on chromosomes in a linear fashion, like beads on a necklace. 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.
Sturtevant's work on the Drosophila genome enabled geneticists to further map chromosomes of higher organisms, including human beings. His former Caltech research partner George Beadle claimed that modern biochemical genetics stems directly from Sturtevant's work.
