Heredity is the passing of genes X-Linked Recessive Disorders 
from one generation to the next. Children inherit their parents' genes. Hereditary metabolic disorders develop when children inherit defective genes that control metabolism. There are different types of inherited disorders Inheritance of Single-Gene Disorders Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body or code for functional RNA molecules.... read more . In most hereditary metabolic disorders, both parents of the affected child carry a copy of the abnormal gene. Because usually two copies of the abnormal gene are necessary for the disorder to occur, usually neither parent has the disorder. Some hereditary metabolic disorders are X-linked X-Linked Recessive Disorders , which means only one copy of the abnormal gene can cause the disorder in boys.
There are hundreds of metabolic disorders and most of them are extremely rare.
Metabolism
Most of the foods and drinks people ingest are complex materials that the body must break down into simpler substances. This process may involve several steps. The simpler substances are then used as building blocks, which are assembled into the materials the body needs to sustain life. The process of creating these materials may also require several steps. The major building blocks are
Metabolism is this complicated process of breaking down and converting the substances ingested.
Metabolism is carried out by chemical substances called enzymes, which are made by cells in the body. If a genetic abnormality affects the function of an enzyme or causes it to be deficient or missing altogether, various metabolic disorders can occur. These disorders usually result from one or both of the following:
Inability to break down a substance that should be broken down, allowing a toxic intermediate substance to build up
Inability to produce some essential substance
Metabolic disorders are classified by the particular building block that is affected.
Diagnosis of Hereditary Metabolic Disorders
Prenatal screening tests
Newborn screening tests
Before birth, some hereditary disorders of metabolism (such as phenylketonuria Phenylketonuria (PKU) Phenylketonuria is a disorder of amino acid metabolism that occurs in infants born without the ability to normally break down an amino acid called phenylalanine. Phenylalanine, which is toxic... read more and lipidoses Overview of Lysosomal Storage Disorders Lysosomal storage disorders are hereditary metabolic disorders. Hereditary disorders occur when parents pass the defective genes that cause these disorders on to their children. There are different... read more ) can be diagnosed in the fetus by using the prenatal screening tests amniocentesis Amniocentesis Prenatal diagnostic testing involves testing the fetus before birth (prenatally) to determine whether the fetus has certain abnormalities, including certain hereditary or spontaneous genetic... read more or chorionic villus sampling Chorionic Villus Sampling Prenatal diagnostic testing involves testing the fetus before birth (prenatally) to determine whether the fetus has certain abnormalities, including certain hereditary or spontaneous genetic... read more . Usually, a hereditary metabolic disorder is diagnosed by using a blood test or examination of a tissue sample to determine whether a specific enzyme is deficient or missing. Genetic testing is also sometimes used.
After birth, many of these disorders are detected by routine newborn screening tests Newborn Screening Tests Screening tests are done to detect health conditions that are not yet causing symptoms. Many serious disorders that are not apparent at birth can be detected by various screening tests. Early... read more . For a complete list of routine newborn screening tests by state, see Conditions Screened by State. However, newborns are not screened for many of the less common hereditary metabolic disorders, and doctors do tests for those disorders only when they suspect a problem.
Doctors may suspect a hereditary disorder of metabolism during a physical examination. Symptoms may provide clues as well. For example, children who have sweet-smelling urine may have maple syrup urine disease Maple syrup urine disease Amino acid metabolism disorders are hereditary metabolic disorders. Hereditary disorders occur when parents pass the defective genes that cause these disorders on to their children. In most... read more , or children who smell like sweaty feet may have isovaleric acidemia Isovaleric acidemia Amino acid metabolism disorders are hereditary metabolic disorders. Hereditary disorders occur when parents pass the defective genes that cause these disorders on to their children. In most... read more . Eye problems, an enlarged liver or spleen, heart problems (such as cardiomyopathy Overview of Cardiomyopathy Cardiomyopathy refers to progressive impairment of the structure and function of the muscular walls of the heart chambers. There are three main types of cardiomyopathy: Dilated cardiomyopathy... read more 
), or muscle weakness may indicate other hereditary metabolic disorders.
More Information
The following are some English-language resources that may be useful. Please note that THE MANUAL is not responsible for the content of these resources.
Conditions Screened by State: This resource helps users in the United States determine which conditions their state screens newborns for.
National Organization for Rare Disorders (NORD): This resource provides information to parents and families about rare diseases, including a list of rare diseases, support groups, and clinical trial resources.
Genetic and Rare Diseases Information Center (GARD): This resource provides and easy to understand information about rare or genetic diseases.
