Exciting times have begun in human and medical genetics. Medical genetics has become recognized as a specialty concerned with the diagnosis, follow-up, and treatment of inherited diseases. The Human Genome Project, which began as an international effort to determine the entire content of the human genome, has been especially important in this field. Together with other branches of modern biology, it has revolutionized human and medical genetics and laid the foundation for better diagnosis, treatment, and preventive approaches for many diseases in the near future.
Genetics deals with heredity and variation in all living organisms. Medical genetics, however, is a branch of human genetics that studies genetic differences important in medical practice and research. It is a unifying concept that connects human genetics with all areas of medicine. For patients and families to benefit fully from expanding genetic knowledge, all doctors and healthcare professionals need to understand the basic principles of human genetics.
The existence of alternative forms of genes in the population, mutations at different locations, the importance of gene–gene and gene–environment interactions, the role of somatic mutations in cancer and aging, prenatal diagnosis, presymptomatic testing, population screening, and the promise of gene therapy are already becoming part of everyday medical practice.
Medical geneticists not only work directly with patients, but also serve society through laboratory diagnosis by identifying individuals who carry genetic diseases or may pass them on to future generations. Medical genetics focuses not only on the patient, but also on the whole family. Taking a detailed family history is the first step in evaluating any disease. Identifying an inherited disease makes it possible to calculate risks for other family members and to provide preventive care and counseling.
In clinical practice, the major importance of genetics is to explain the role of mutations and genetic differences in the causes of many diseases. Almost all diseases result from an interaction between genes and the environment, although the relative contribution of genetics may be small or large.
Diseases partly or entirely caused by genetic factors can be divided into three groups:
- Single-gene disorders
- Chromosomal disorders
- Multifactorial disorders
Single-gene disorders are caused by a mutation in one specific gene. Although many of these diseases are individually rare, together they account for an important proportion of illness and death. Chromosomal disorders occur when there is an extra or missing chromosome or chromosome fragment. For example, an extra chromosome 21 causes Down syndrome. Multifactorial diseases are caused by a combination of small genetic differences and environmental factors.
Today, genetic laboratories are rapidly advancing in areas such as diagnosis of genetic diseases, carrier testing, prenatal diagnosis, and identifying individuals who may develop disease later in life. Population screening for genetic diseases is also becoming increasingly widespread.
Important areas of specialization include cytogenetics, molecular genetics, genomics, population genetics, and clinical genetics. In addition, genetic counseling has emerged as a new field that provides patients and families with information about genetic risks, along with psychological and educational support.