Genetics is unique among the various disciplines in medicine because of its focus on a familiar rather than on an individual patient. Medical genetics is concerned not only with making the correct diagnosis in a particular case but with determining the genotypes of other family members and estimating recurrence risks both for the parents of an affected person and for his or her sibs, as well as for more distant relatives. Furthermore, because such risks are usually affected not only by the genotypes of persons from the general population who have married into the family, genetic counselling must take into account like livelihood of specific genotypes in different populations. Thus in order both to make a genetically correct clinical diagnosis and to determine recurrence risks as part of effective genetic counselling, it often matters , for example, whether a family has its origins in the British Isles or in the Mediterranean or in Finland, whether the family is Caucasian, Black, or Asian. In genetics more than in any other medical speciality, the patient is a reflection of the population to which he or she belongs.
Population Genetics is the study of the distribution of genes in population and of how the frequencies of genes and genotypes are maintained or changed. Population genetics has much in common with epidemiology, the study of the interrelationships of the various genetic and evnironmental factors that determine the frequency and distribution of diseases in human communities. The two areas fuse in the field of genetic epidemiology, which is chiefly concerned with diseases that have complex patterns of inheritance or are caused by a combination of heritable and environmental factors. This approach has already improved our understanding of the genetics of many diseasesm especially the common disorders of adult life.