### Young: Medical Genetics

# Risk Calculations Questions

Risk calculation tends to feature prominently in undergraduate examinations, either as multiple choice questions or as short answer notes which may require the construction of a family tree followed by risk assessment. Generally these questions are relatively easy to set and mark, so it is not surprising that they are popular with examiners. With a little understanding they are usually equally easy to answer. In addition they have the advantage for students that they do not require extensive knowledge of genetics but simply a grasp of basic principles and the ability to apply a little logic. The examples that follow illustrate the range of topics which can arise and are graded in increasing order of difficulty. Remember that risks (probability) can be expressed in several different ways, i.e. as a fraction (1 in 2), as a percentage (50%) or as a proportion of 1 (0.5).

It is recommended that before working through these examples, reference is made to the sections in Chapters 4, 7 and 13 dealing with the use of linked markers (p 91-93), Hardy Weinberg equilibrium (p 136-138) and basic approaches to risk calculation (p 136-138). Those with a masochistic penchant for this type of exercise might like to consult a copy of the author's textbook entitled *Introduction to Risk Calculation in Genetic Counseling* — 3rd edition to be published by Oxford University Press, New York in 2006 — but note that this book is expensive and generally much too complicated for undergraduates, so only consult it if you are mathematically inclined and your local library has an available copy!