People love to oversimplify genetics, saying we have a ‘gene for cancer’ or a ‘gene for diabetes’. But the fact is, genes determine only so much. Identical twins are identical genomes, yet one may develop juvenile diabetes and the other typically doesn't. Understanding the role of genes should help pinpoint environmental factors … The genome is a history book showing the entire 6 billion-member human species traces back 7000 generations to a tiny founding population of some 60 000 people. Our species has only a modest amount of genetic variation—the DNA of any two humans is 99.9% identical.
ERIC LANDER, HUMAN GENOME PROJECT, 20001
The family doctor has an important role to play in the exciting and rapidly expanding world of medical genetics. The role includes routine diagnosis, early detection, and community and ethical guidance. Virtually all of the three billion nucleotides of the human genome have been sequenced and the knowledge of their organisation into the known 30 000–35 000 functional units or genes continues to become more sophisticated.2
The genome project has commenced mapping out ‘single nucleotide polymorphisms’ (SNPs) as signposts throughout the genome to assist in locating disease-associated genes and studying variations between individuals.3 Any two unrelated individuals differ by one base per every thousand or so—these as SNPs—and it is believed that SNPs contribute to the risk of common disease rather than directly cause disease. If we carry the wrong set of SNPs, we can be predisposed to various diseases.
Genetic testing is now available for many common hereditary disorders, such as the HFE genes for haemochromatosis, presymptomatic DNA tests are available for the hereditary neurological disorders, such as Huntington disease, and predictive DNA testing is available for some forms of hereditary cancer, such as breast and colon cancer, and in the future for cardiovascular disease and diabetes.4 Also in the future, pharmacogenetics, which predicts genetically determined responses to pharmaceuticals, will greatly assist rational prescribing, and gene therapy is a futuristic treatment modality.5
An important recent development in gene expression has been advances in DNA technology, with the detection of fetal DNA in maternal plasma, thus allowing an excellent screening test for Down syndrome.6
Epigenetics, meaning ‘on top of’ traditional genetic inheritance, is a study of changes in genetic expression or cellular phenotype caused by mechanisms other than changes in the DNA sequence. These factors, which include environment, lifestyle, nutrition and psychosocial influences, can influence the outcomes of chronic illness such as cancer, diabetes, autoimmune disorders and ageing. The incurred changes are heritable. This is an expanding development.
A summary of the prevalence of genetic disease is presented in TABLE 18.1.
Table 18.1Prevalence of genetic disease (after Kingston)7