The underlying philosophy of the First Edition was that the teaching of genetics should begin with DNA rather than Mendel. Nothing has happened during the intervening 3 years to change my mind about the molecular approach: if anything I am more convinced than ever that an initial understanding of the gene as a piece of DNA provides the student with the confidence needed to deal successfully with the challenges and subtleties of the more 'classical' aspects of genetics. The Second Edition therefore retains the molecular approach, although with two important differences. The first is that my own confidence has been boosted to the extent that I have now taken the narrative slightly further, in an attempt to provide a more thorough introduction for degree programmes in which genetics will form a large part of the subsequent coursework. To this end the existing sections on gene analysis have been expanded and additional topics such as population genetics and evolution brought in at appropriate places. These changes make the book more complete in its coverage and should not detract from its popularity as a concise introductory text for the genetics component of general biology courses. The second difference is that I have given eukaryotes rather more emphasis, especially in Part One. There has always been a temptation to base an intro ductory series of molecular biology lectures solely on E.