The gene theory was developed by Gregor Mendel, an Austrian Augustinian friar and scientist, through his work on pea plants in the mid-1800s. Mendel's experiments and observations laid the foundation for the field of genetics. He identified the basic principles of heredity and proposed the concept of genes as discrete units that determine inherited traits. His work contributed significantly to our understanding of how traits are passed down from parents to offspring and established the groundwork for modern genetics.