Having now considered the chemical structure of DNA and how DNA is transcribed into RNA, we can ask: What is a gene? As noted in Chapter 3, the definition of a gene often changes as we explore different aspects of heredity. A gene was defined in Chapter 3 as an inherited factor that determines a characteristic. This definition may have seemed vague because it says only what a gene does rather than what a gene is. Nevertheless, this definition was appropriate at the time because our focus was on how genes influence the inheritance of traits. We did not have to consider the physical nature of the gene in learning the rules of inheritance.

At this point, we can be more precise about what a gene is. Chapter 8 described how genetic information is encoded in the base sequence of DNA; a gene consists of a set of DNA nucleotides. But how many nucleotides constitute a gene, and how is the information in those nucleotides organized? In 1902, Archibald Garrod correctly suggested that genes encode proteins. Proteins are made of amino acids, so a gene contains the nucleotides that specify the amino acids of a protein. Therefore, for many years, the working definition of a gene was a set of nucleotides that specifies the amino acid sequence of a protein. As geneticists learned more about the structure of genes, however, it became clear that this concept of a gene was an oversimplification.