the Ancient Greek: γόνος, gonos, that means offspring and procreation. Definitions There are many different ways to use the term "gene" based on different aspects of their inheritance, selection, biological function, or molecular structure but most of the definitions fall into two categories, the Mendelian gene or the molecular gene. The Mendelian gene is the classical gene of genetics and it refers to any heritable trait. This is the gene described in "The Selfish Gene." More thorough discussions of this version of a gene can be found in the articles on Genetics and Gene-centered view of evolution. This article focuses on the molecular gene—the gene that's described in terms of DNA sequence. There are many different different definitions of this gene - some of which are mispleading or incorrect. . The very first edition of the textbook "Molecular Biology of the Gene" (1965) described two kinds of molecular gene: protein-coding genes and those that specified functional RNA molecules such as ribosomal RNA and tRNA (noncoding genes). But the idea of two kinds of genes dates back to the late 1950s when Jacob and Monod speculated that regulatory genes might produce repressor RNAs. This idea of two kinds of genes is still part of the definition of a gene in most textbooks. For example, "The primary function of the genome is to produce RNA molecules. Selected portions of the DNA nucleotide sequence are copied into a corresponding RNA nucleotide sequence, which either encodes a protein (if it is an mRNA) or forms a 'structural' RNA, such as a transfer RNA (tRNA) or ribosomal RNA (rRNA) molecule. Each region of the DNA helix that produces a functional RNA molecule constitutes a gene." "We define a gene as a DNA sequence that is transcribed. This definition includes genes that do not encode proteins (not all transcripts are messenger RNA). The definition normally excludes regions of the genome that control transcription but are not themselves transcribed. We will encounter some exceptions to our definition of a gene - surprisingly, there is no definition that is entirely satisfactory." "A gene is a DNA sequence that codes for a diffusible product. This product may be protein (as is the case in the majority of genes) or may be RNA (as is the case of genes that code for tRNA and rRNA). The crucial feature is that the product diffuses away from its site of synthesis to