Genes are viewed both as units of heredity in the classical sense and as segments of DNA with specific molecular functions in the modern perspective. The main focus of the manuscript is to demonstrate how the principles of classical genetics integrate with the molecular understanding of genes, DNA, and heredity. It explains the principles of heredity and gene function, combining traditional Mendelian concepts with modern molecular biology to show how genetic information is inherited, expressed, and regulated. The core learning objectives include understanding the foundations of classical genetics, recognizing the physical nature of genes as DNA sequences, linking classical principles to molecular mechanisms, explaining genetic processes at the molecular level, and connecting genetics with modern applications. The author presents these fundamentals with up-to-date examples, while deliberately avoiding exhaustive detail.