Sturtevant, Alfred Henry (1891–1970)

US geneticist who was the first, in 1911, to map the position of genes on a chromosome. He worked with US biologist Thomas Morgan.

Mutant fruit flies In 1903, Sturtevant began his seminal experiments on the genetics of the fruit fly Drosophila melanogaster. In May 1910, Sturtevant and Morgan discovered a mutated Drosophila with a white eye, instead of the normal red. Sturtevant's first breeding experiments were with white-eyed and red-eyed flies. He also developed other mutations using genotoxic agents such as X-rays.

Using these variant flies, Sturtevant developed methods for mapping gene positions on chromosomes. In 1911, he produced the first gene map ever derived, showing the positioning of five genes on a Drosophila X chromosome: white-eyed, vermilion-eyed, rudimentary wings, small wings, and yellow body.

Determining the gene order Sturtevant and Morgan determined the gene order along the chromosome by working out the recombination frequencies between the linked genes. During meiosis (cell division that produces reproductive cells with half the number of chromosomes) in a heterozygous, phenotypically normal female, an X chromosome carrying the mutations and a normal X chromosome will rearrange or ‘crossover’. Males derived from these females will carry a combination of mutations. Those mutations that are closest together on the chromosome will be seen with the greatest frequency in the phenotypes (traits) of the male flies.

Inheritance of these mutations in flies was not strictly according to Mendel's rules since female flies had to inherit two mutated X chromosomes to demonstrate the phenotype, whereas males only had to acquire a single mutated X chromosome. This made the discovery that the five genes were on the same chromosome striking.