Crosses are made by placing a male and a female Drosophila melanogaster in a single milk bottle; the labels record the genotypes of the flies in the cross. Note that Sturtevant is wearing a suit & tie, and smoking a pipe in a room where ether was used to anesthetize flies for counting offspring numbers and arranging crosses.

    Alfred Henry Sturtevant (1891 – 1970) was an American geneticist whose first genetic map helped establish the chromosome theory of inheritance. In 1910, he joined Thomas Hunt Morgan’s laboratory at Columbia University as an undergraduate, alongside Calvin Bridges and Hermann Muller. In 1913, he published the first genetic map, which described the positions of genes on the X chromosome of Drosophila melanogaster. Whereas Mendel had advocated the principle of Independent Assortment of gene loci, Sturtevant showed that the gene loci could co-occur on the same chromosome, and that measurement of recombination frequencies could reveal the order and relative distances of genes along a chromosome. In 1914, he received his doctorate from Columbia.

    In 1925, Sturtevant demonstrated unequal crossing over at the Bar locus, and he showed that a gene’s chromosomal position could affect its expression. In 1928, he moved with Morgan and Bridges to the California Institute of Technology, as Professor of Genetics. His research extended across developmental genetics, chromosome evolution, and Drosophila taxonomy. With Theodosius Dobzhansky, he used chromosome inversions to trace evolutionary relationships among fruit-fly populations. He also helped establish the principles of genetic fate maps, which connected patches of tissue in the embryo with their adult organs. His 1965 A History of Genetics drew on his firsthand knowledge of the discipline’s early development.