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Genetic disorders and birth defects are sometimes perceived as being so uncommon that the general pediatrician will seldom encounter them. However, each day more than 400 babies with birth defects are born in the United States, and 1 of every 5 infant deaths is caused by these disorders. Virtually every medical condition, except for some cases of trauma, is influenced by an individual’s genetic background. Rapid advances in our knowledge of the genetic basis of disease, coupled with emerging technologies for high-throughput genetic testing, are also rapidly transforming the practice of medicine.


Comprehensive databases of factual information about the identification of new disease genes, genotype–phenotype correlation, and the availability of genetic testing have become widely available online and in published review articles. Thus, each of the following sections seeks to emphasize important fundamental concepts while providing paradigmatic examples.


Traditionally, various human traits (eg, skin color, intelligence) and disorders (eg, phenylketonuria, achondroplasia) have been divided into those that are genetic versus those that are nongenetic, the latter usually referring to disorders “determined” by the environment (eg, infectious diseases, teratogens). However, this dichotomy separating gene and environment is largely artificial, and the distinction between the two has become increasingly blurred. Disorders caused by mutations in a single gene (eg, cystic fibrosis, sickle-cell disease) can be heavily influenced by the environment, and diseases caused by the environment (eg, AIDS secondary to infection with HIV-1) can be substantially modified by the presence of certain polymorphisms. In other words, the cause of most diseases can be considered genetic and environmental; some diseases are more strongly influenced by genes, whereas others are more strongly influenced by the environment.


Whether defined narrowly or broadly, genetic disorders and birth defects contribute substantially to morbidity and mortality in pediatric patients. For example, the percentage of deaths attributable to genetic disease in hospitals in the United Kingdom has risen from 16.5% in 1914 to 50% in 1976. Birth defects are also the leading cause of death in infancy in the United States, accounting for more than 20% of infant deaths. Cardiovascular malformations are the leading cause of premature mortality from congenital anomalies. This reflects a better understanding of the etiology of pediatric diseases as well as the substantial reduction in infectious disease and perinatal mortality during the 20th century. Population-based studies suggest that birth defects and genetic disorders account for about 10% of hospitalizations and about 30% of all hospitalization charges. Approximately 7% of pediatric admissions are for single-gene and chromosomal disorders, and another 15% to 20% for congenital malformations of different types. Moreover, about 35% of hospital deaths of children are caused by a genetic condition and/or birth defect. Recent advances in therapeutics have substantially prolonged the survival of children with birth defects and/or genetic diseases, challenging practitioners to care for older children and young adults with genetic disorders.


Classifications of genetic disorders depend, in part, on the delineation of a phenotypic trait (eg, ...

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