Gregor Mendel was an Austrian monk who lived at the end of the last century. He worked on breeding pea plants in the garden of his monastery. He developed an idea that the information on how to build of each part of the plant came from a single piece of information passed on from one of its parents. When Gregor's work was rediscovered 20 years after his death, at the beginning of this century, this unit of information became known as a gene.
The gene was just an idea. It referred to a piece of information not the physical object itself. People knew that it had to be in every living cell but no one knew which part of the cell actually carried this information. The prime candidates were Proteins or Nucleic Acids because they are complicated enough to do the job and they are present in all cells. By the middle of the 1940's scientists gained strong evidence that showed that genes were made up from Nucleic Acids, specifically DNA.