This book reviews key advances and new fundamentals in genetics. The increasing importance of genetic approaches in diverse areas of biology and medical sciences constantly requires in-depth information on genetic discoveries and research strategies for advanced graduate-level students as well as current researchers. This book focuses on genetic studies of various animal model systems and their major contributions to establishing modern genetics. Information covered in this book is mostly based on original research papers that extend from classical to modern genetics and applications. The contents are organized into four parts. Part I introduces fundamental concepts and experimental strategies in classical genetics. Part II discusses molecular genetics with transposons, transgenesis, clonal analysis, and gene editing technologies. Part III emphasizes epigenetic regulation of genome organization and gene expression. Part IV integrates earlier parts with landmark genetic studies on non-coding RNAs in dosage compensation, programmed cell death, growth control related to cancer, and behavioral neurobiology.