ABSTRACT
Medical genetics traditionally deals with clinical conditions purely or largely due to genetic causes such as single-gene (Mendelian) disorders. More than 4500 Mendelian disorders are known but they constitute only 1% of all diseases. Information used by medical geneticists to identify these rare diseases has strong implications for genetic association study design, execution, and interpretation. Most current genetic association studies are typically for multifactorial or complex disorders, which are determined by the interplay among multiple gene variants and environmental factors, but the principles of medical genetics are important in understanding the nature of multifactorial disorders and setting up informative studies. It should be noted that mutations causing Mendelian disorders were located using linkage studies, which are not covered in this book. This chapter will review the main principles of medical genetics and how they connect to the study of multifactorial disorders by association studies.
