[Genetics for the non-geneticist. Semantics and basic facts]

This chapter is an introduction to a new language, that of genetics. The first part recalls and illustrates the elementary concepts of molecular genetics, with emphasis on the structure of DNA and the chromosomes, that of genes, the genetic code and the genetic factors of sexual differentiation. The second part explains medical molecular genetics which is based on a fundamental concept: polymorphism of DNA, a normal and physiological phenomenon which arises mostly from genetic exchanges which occur during meiosis. Monogenic diseases with mendelian transmission may be autosomal dominant or recessive or linked to the X-chromosome. The understanding of multigenic diseases requires the use of much more complex techniques based on so-called "reverse" genetics or on the "candidate gene" method.