In the 1930s, George Beadle and Boris Ephrussi discovered factors that affect eye colors in developing fruit flies. They did so while working at the California Institute of Technology in Pasadena, California. (1) They took optic discs (colored fuchsia in the image) from fruit fly larvae in the third instar stage of development. Had the flies not been manipulated, they would have developed into adults with vermilion eyes.

Johann Gregor Mendel studied patterns of trait inheritance in plants during the nineteenth century. Mendel, an Augustinian monk, conducted experiments on pea plants at St. Thomas’ Abbey in what is now Brno, Czech Republic. Twentieth century scientists used Mendel’s recorded observations to create theories about genetics.

Sanger sequencing is a laboratory method that scientists use to determine the sequence of nucleotides within a piece of deoxyribonucleic acid, or DNA. Frederick Sanger, who studied biochemistry in the United Kingdom, and colleagues developed the technique in 1977. Sanger sequencing was one of the first methods to sequence long stretches of DNA accurately, allowing researchers to collect and analyze large amounts of genetic data. The technique uses modified nucleotides that terminate DNA synthesis at specific points, which results in fragments of varying lengths that researchers sort to determine the sequence of nucleotides. Sanger sequencing was one of the most widely used sequencing methods for thirty years after its creation and facilitated early genomic sequencing projects, such as the Human Genome Project, or HGP. As of 2025, Sanger sequencing remains a common technique despite the advent of newer sequencing technologies and plays a role in clinical settings and genetic counseling, a service that provides information on genetic conditions to people and their families.

Theodosius Dobzhansky studied genetics and evolutionary biology, primarily using fruit flies, in the Soviet Union and the United States during the twentieth century. Scholars credit his widely read 1937 book Genetics and the Origin of Species with providing one of the earliest and most impactful integrations of Charles Darwin’s theory of evolution by natural selection with Gregor Mendel’s theory of genetics and therefore contributing significantly to the Modern Synthesis. In his research, Dobzhansky studied wild populations of organisms such as fruit flies and found that populations of the same species can differ in the frequency of one or more genetic traits. Understanding the role of such genetic diversity in evolution became a central theme of his research. Later in his career, he wrote about the social implications of his research, including the relationship between biological and cultural notions of race. Through both his scientific papers and popular writings, Dobzhansky shaped the course of evolutionary biology in the twentieth century and influenced public discussions of race in the years following World War II.