ABSTRACT
The disorders to be considered, in contrast to most Mendelian conditions, will be encountered frequently by those working in all branches of medicine and in primary care. Genetic testing for Mendelian subsets of the common disorders has proved fruitful in clinical practice, both for the high-risk Mendelian contexts and also for reassurance and the avoidance of unnecessary interventions in family members at low risk. The term ‘heritability’ is often encountered in relation to common disorders and refers to the proportion of variation of all types, familial and other, that is determined by genetic factors, regardless of any pattern of inheritance. The essential distinguishing factor from the Mendelian disorders is that a single genetic locus cannot be held responsible for the condition; it is the result of the additive effects or interactions of a number of genetic loci and often a number of external factors.
