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Genetics is the branch of science that deals with genes, heredity, and the variation of inherited characteristics. Medical genetics is the study of the etiology, pathogenesis, and natural history of human diseases that are at least partially genetic in origin. Diagnosis, management, and prevention of such diseases are also part of this field.

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Genetic disease is common. Between 2 and 3 percent of newborns have a recognized structural defect, another 3 percent have a defect diagnosed by age 5, and by age 18, another 8 to 10 percent are discovered to have one or more functional or developmental abnormalities. Added to these is the susceptibility to many common diseases that have a genetic basis as well as most cancers, which develop as the result of cumulative mutations. All in all, an astounding two thirds of the population will experience a disease with a genetic component in their lifetimes. Tests available for the prenatal diagnosis of selected genetic diseases and the rationale accompanying their use are described throughout Chapter 13.

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The Human Genome Project, supported principally by the National Human Genome Research Institute, accomplished complete sequencing of the human genome in 2003. Efforts have since focused on genomics, the study of functions and interactions of all genes, to better understand the biology of disease (Burke, 2003; McKusick and Ruddle, 2003). The Online Mendelian Inheritance in Man—OMIM was developed in 1985 by the National Library of Medicine in collaboration with Johns Hopkins University. In 1995, it was developed for the World Wide Web by the National Center for Biotechnology Information. It is an up-to-date compendium of human genes and phenotypes. As of April 2009, the OMIM listed more than 12,700 unique genes with known sequence. Of these, greater than 12,000 are autosomal dominant or recessive, more than 600 are X- or Y-linked, and 37 are mitochondrial. OMIM has also catalogued more than 2800 mendelian conditions that either have a known molecular basis or have an identified responsible gene that has been sequenced. And finally, the catalogue has more than 1500 conditions for which the molecular basis remains unknown. In the future, it is likely that consideration of individual genetic background and specific traits and susceptibilities will enhance routine medical care. From the foregoing, it is obvious that knowledge of genetic principles is essential for all clinicians.

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The 22 pairs of autosomes and one pair of sex chromosomes can be affected by a wide range of numerical and structural abnormalities that profoundly affect gene expression.

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Standard Nomenclature

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Karyotypes are reported using nomenclature agreed upon by the genetics community and codified as the International System for Human Cytogenetic Nomenclature (ISCN, 2009). Karyotypic abnormalities fall into two broad categories—those of chromosome number, such as trisomy, and those of chromosome structure, such as a translocation or deletion. Each chromosome has a short arm—the “p” or petit arm and ...

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