Genetics is playing an increasingly important role in the practice of clinical medicine. Medical genetics, once largely confined to relatively rare conditions seen by only a few specialists, is now becoming a central component of our understanding of most major diseases. These include not only the pediatric diseases, but also common adult diseases such as heart disease, diabetes, many cancers, and many psychiatric disorders. Because all components of the human body are influenced by genes, genetic disease is relevant to all medical specialties. Today’s health-care practitioners must understand the science of medical genetics.
What Is Medical Genetics?
Medical genetics involves any application of genetics to medical practice. It thus includes studies of the inheritance of diseases in families, mapping of disease genes to specific locations on chromosomes, analyses of the molecular mechanisms through which genes cause disease, and the diagnosis and treatment of genetic disease. As a result of rapid progress in molecular genetics, DNA-based diagnosis is available for several thousand inherited conditions, and gene therapy—the alteration of genes to correct genetic disease—is now effective for some conditions. Medical genetics also includes genetic counseling, in which information regarding risks, prognoses, and treatments is communicated to patients and their families.
Why Is a Knowledge of Medical Genetics Important for Today’s Health-Care Practitioner?
There are several reasons health-care practitioners must understand medical genetics. Genetic diseases make up a large percentage of the total disease burden in pediatric and adult populations ( Table 1.1 ). This percentage will continue to grow as our understanding of the genetic basis of disease grows. In addition, modern medicine is placing increasing emphasis on prevention. Because genetics provides a basis for understanding the fundamental biological makeup of the organism, it naturally leads to a better understanding of the disease process. In some cases, this knowledge can lead to prevention of the disorder. It also leads to more effective disease treatment. Prevention and effective treatment are among the highest goals of medicine. The chapters that follow provide many examples of the ways genetics contributes to these goals. But first, this chapter reviews the foundations upon which current practice is built.
| Disease | Approximate Prevalence |
|---|---|
| Chromosome Abnormalities | |
| Down syndrome | 1/700 to 1/1000 |
| Klinefelter syndrome | 1/1000 males |
| Trisomy 13 | 1/10,000 |
| Trisomy 18 | 1/6000 |
| Turner syndrome | 1/2500 to 1/10,000 females |
| Single-Gene Disorders | |
| Adenomatous polyposis coli | 1/6000 |
| Adult polycystic kidney disease | 1/1000 |
| α-1 Antitrypsin deficiency | 1/2500 to 1/10,000 (whites) ∗ |
| Cystic fibrosis | 1/2000 to 1/4000 (whites) |
| Duchenne muscular dystrophy | 1/3500 males |
| Familial hypercholesterolemia | 1/500 |
| Fragile X syndrome | 1/4000 males; 1/8000 females |
| Hemochromatosis (hereditary) | 1/300 whites are homozygotes; approximately 1/1000 to 1/2000 are affected |
| Hemophilia A | 1/5000 to 1/10,000 males |
| Hereditary nonpolyposis colorectal cancer | Up to 1/200 |
| Huntington disease | 1/20,000 (whites) |
| Marfan syndrome | 1/10,000 to 1/20,000 |
| Myotonic dystrophy | 1/7000 to 1/20,000 (whites) |
| Neurofibromatosis type 1 | 1/3000 to 1/5000 |
| Osteogenesis imperfecta | 1/5000 to 1/10,000 |
| Phenylketonuria | 1/10,000 to 1/15,000 (whites) |
| Retinoblastoma | 1/20,000 |
| Sickle cell disease | 1/400 to 1/600 blacks ∗ in America; up to 1/50 in central Africa |
| Tay-Sachs disease | 1/3000 Ashkenazi Jews |
| Thalassemia | 1/50 to 1/100 (South Asian and circum-Mediterranean populations) |
| Multifactorial Disorders | |
| Congenital Malformations | |
| Cleft lip with or without cleft palate | 1/500 to 1/1000 |
| Club foot (talipes equinovarus) | 1/1000 |
| Congenital heart defects | 1/200 to 1/500 |
| Neural tube defects (spina bifida, anencephaly) | 1/200 to 1/1000 |
| Pyloric stenosis | 1/300 |
| Adult Diseases | |
| Alcoholism | 1/10 to 1/20 |
| Alzheimer disease | 1/10 (Americans older than 65 years) |
| Bipolar disorder | 1/100 to 1/200 |
| Cancer (all types) | 1/3 |
| Diabetes (types 1 and 2) | 1/10 |
| Heart disease or stroke | 1/3 to 1/5 |
| Schizophrenia | 1/100 |
| Mitochondrial Diseases | |
| Kearns-Sayre syndrome | Rare |
| Leber hereditary optic neuropathy (LHON) | Rare |
| Mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) | Rare |
| Myoclonic epilepsy and ragged red fiber disease (MERRF) | Rare |
∗ The term “white” refers to individuals of predominantly European descent; the term “black” refers to individuals of predominantly sub-Saharan African descent. These terms are used for convenience; some of the challenges in accurately describing human populations are discussed in Chapter 14 .
A Brief History
The inheritance of physical traits has been a subject of curiosity and interest for thousands of years. The ancient Hebrews and Greeks, as well as later medieval scholars, described many genetic phenomena and proposed theories to account for them. Many of these theories were incorrect. Gregor Mendel ( Fig. 1.1 ), an Austrian monk who is usually considered the father of genetics, advanced the field significantly by performing a series of cleverly designed experiments on living organisms (garden peas). He then used this experimental information to formulate a series of fundamental principles of heredity.
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