Basic principles of genetics and genetic counselling.

Genetic counselling and testing is a fast growing field in modern medicine. Genetic analysis can aid in the diagnosis, treatment, management and prevention of (genetic) diseases in patients. This review provides an overview of the fundamental principles of genetics from a clinical perspective, focusing on the practical application and interpretation of genetic testing. Key concepts, including genetic expression and inheritance patterns, are discussed to give a better understanding of the subject. There is a large variety of genetic testing methodologies, from single gene testing to whole genome sequencing (WGS) that have their respective advantages, disadvantages and clinical implications. Interpretation of genetic results is performed on different levels: from the molecular level (variant interpretation and classification) to the clinical level (association of genetic variants with disease and phenotype). The correct interpretation of the results is essential to use genetics for clinical decision making. Challenges in the field include, but are not limited to, variants of unknown significance, incidental findings, and the familial forms of disease that remain gene-elusive with the current technologies. Pre- and post-test genetic counselling is essential to ensure that patients and their family members are aware of the options and implications of genetic testing, which will significantly benefit patients with coping. This review is based on the latest European and American guidelines, to give an up-to-date overview of rapid advancements of genetic testing and counselling. A dedicated follow-up article in this series will explore the Indian perspective in greater depth.