Inheritance concerns the exact transmission of genetic information from parents to their progeny. Variability of the inherited characteristics can be accounted for by a change either in a genetic makeup of a cell or in environmental conditions. Deoxyribonuclic acid is responsible for heredity in all cells. It was identified as a compound bearings genetic information whwn in 1949 Avery, Maclead and McCarty discovered that a non-virulent strain of the bacterium Streptococcus Pneumoniae could be transformed in a heritable manner into a virulent strain by simply adding DNA extracted from a dead virulent strain in to the medium.