Medical genetics, a branch of human genetics, spans a wide spectrum of sub-branches including basic, clinical, diagnostic, therapeutic, prophylactic and applied genetics. This booklet, I hope it be the first in a series, is confined to two disciplines of basic genetics, viz. molecular genetics and pathogenetics, that are concerned with studying the structure and function of the genetic material in health and disease states. It aims at offering the basic concepts of these two subjects to clinicians and research workers in the field. For clinicians beginning their specialization as medical geneticists, I tried my best to offer, as simplified as possible, definitions and illustrations of the main points of both subjects concerning the structure and function of genes, mutagens and disease-causing mutations, as well as the anti-mutation mechanisms of the human genome. For research workers, I pointed out some of the most enigmatic aspects of the field that, still, await more research to disclose in order to go a step further in our way to understand what we think we know about the role of our genetic constitution in shaping our life.