Wilhelm Johannsen coined the term "gene" in 1909. However, before him came scientists such as Gregor Mendel, Hugo de Vries, Carl Correns and Erich von Tschermak who all developed theories on inheritance patterns in plants. They all understood that traits could be inherited, but they did not visualize genes or have evidence of their existence.
The discovery of genes came from many different scientists over many different years. Each scientist expanded upon the work of scientists who came before them to delve more into the idea of genetics. It wasn't until the late 19th century and early 20th century that chromosomes and genes were first observed.