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Understanding the influence of genetics on health and disease has been a major goal in scientific research over the last century. A landmark of this initiative is the Human Genome Project, which has sequenced the 3 billion base pairs of DNA to provide a road map for studying the 30,000 to 40,000 human genes.1

These advances have been accompanied by an increasing realization that human health—including diseases and conditions associated with pediatric hospitalization—is deeply influenced by genetics. In classic studies, 0.4% to 2.5% of pediatric hospitalizations were attributable to chromosomal abnormalities, and 6% to 8% were attributable to single gene defects; another 22% to 31% of diseases were considered to have a genetic factor.2,3 Since then, as more diseases have been shown to have genetic components, these numbers have risen. A recent report suggests that 71% of pediatric admissions involve a significant genetic component, and 34% of disorders with a clear genetic basis result in 50% of annual hospital costs.4

In this chapter, we review the major pediatric genetic diseases caused by abnormalities that affect a whole chromosome or segment of a chromosome. In addition, clinical situations in which genetic testing should be considered and the types of testing used are discussed.


The essential tools that the clinical geneticist uses to diagnose genetic disorders include the following:

  • Prenatal, birth, and medical histories

  • Detailed pedigree analysis

  • Careful clinical evaluation, including dysmorphology examination

  • Comprehensive literature searches

  • Cytogenetic and molecular genetic laboratory analyses

The generalist can also use many of these tools, although effective use of some testing modalities may require input from a clinical geneticist.

Key features of the family history include ethnic origin, consanguinity, related disorders in the family, and pregnancy losses. The dysmorphology examination is intended to reveal and quantify abnormal development of organs or other parts of the body, as outlined in Table 84-1.

TABLE 84-1Elements of the Dysmorphology Examination

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