A gene is the molecular unit of heredity of a living organism. It is used extensively by the scientific community as a name given to some stretches of deoxyribonucleic acids (DNA) and ribonucleic acids (RNA) that code for a polypeptide or for an RNA chain that has a function in the organism, though there still are controversies about what plays the role of the genetic material. DNA (deoxyribonucleic acid) is a biomolecule that holds the blueprint for how living organisms are built. Gene duplications and chromosomal rearrangements differentiate multiple genomes taken from a single species and genomes from different species. This encyclopaedia is suitable for use as a text in similar overview courses about genes and social issues or genes and disease.