education; laboratory quality control; and social, legal, and ethical issues—can be found in later chapters of the report. Genetic disorders are discussed because they illustrate issues in genetic testing; the list of genetic disorders is not intended to be encyclopedic.
Genetics explores the manner by which specific traits are passed from generation to generation and how they are expressed. Genetics can be studied at many levels. For example, study of an individual's phenotype, or observable properties, can provide information about modes of inheritance, allowing estimates for risks of recurrence. Studies of an individual's chromosomes, or cytogenetics, provide information about the person's gender and about certain diseases that are directly related to abnormal numbers or configurations of the 23 pairs of chromosomes found in humans (e.g., Down syndrome, fragile X syndrome). Genetic testing may involve studies of a physiological, immunological, or biochemical function, or may involve direct study of the genes in the individual's genome. Assessment of the molecular basis for inheritance is done by examining the specific structure and function of genetic material, or DNA. Locating a disease-causing gene on a chromosome and isolating it are an important goal of research. Elucidating the gene's structure and function may provide opportunities for diagnosis and may lead to treatment of the disorder. Molecular biology is being integrated into genetics and medicine at a rapid pace.
Understanding the associations between a gene's information and the physical manifestation of its instructions is accomplished by studies of gene expression (i.e., how the organism carries out the instructions of the DNA to create products that are essential for structure and function of all cells in the body). Understanding gene expression and its regulation is the key to understanding genetic disease and hereditary variation. Hereditary variation is the result of changes—or mutations—in DNA.
Changes that occur in germ cells (egg or sperm) are inherited by offspring. Changes that occur in somatic cells (body cells other than egg or sperm) are not passed to future generations but can result in disease for the individual possessing them (e.g., cancer). Changes—sometimes called mutations—can occur as a result of mistakes in coding in the coding nucleotides, rearrangements within the gene, insertion of new genetic material into the gene, or duplication or deletion of parts or all of a gene. Disorders resulting from changes in one gene alone are called monogenic (e.g., cystic fibrosis, sickle cell anemia, Duchenne muscular dystrophy). Disorders resulting from changes in several genes, usually in combination with an environmental influence, are called multifactorial. Multifactorial disorders (e.g., common types of coronary heart disease and most forms of diabetes) tend to affect far more individuals than do monogenic disorders.
In human monogenic disorders, the altered gene can be located on any one of