Book excerpt: Genetic counselling is widely accepted as an integral part of the management of every patient with an inherited disorder. This book describes in detail how risks can be estimated or calculated for most counselling situations, using nothing more complex than a knowledge of the simple laws of probability and a pocket calculator. All patterns of inheritance are considered. Particular attention is paid to the segregation of balanced chromosome rearrangements and risk calculation in single gene inheritance taking into account factors such as reduced penetrance, late age of onset, consanguinity and the extended family. The use of linked markers -- an area of rapidly increasing importance in carrier detection and prenatal diagnosis -- is also covered at length. The text has been written in a straightforward, "user-friendly" style with liberal use of worked examples and will be of benefit to everyone involved in the provision and delivery of genetic counselling services.