In Williams' irst edition of Obstetrics, he rarely referenced inherited conditions that Gregor Mendel had described 50 years earlier. Fast-forward to 2017, when the science of genetics is a major obstetrical discipline.
Genetics is the study of genes, heredity, and the variation of inherited characteristics. Medical genetics deals with the etiology and pathogenesis of human diseases that are at least partially genetic in origin, along with their prediction and prevention. Thus, it is closely linked to genomics, which is the study of gene function and interaction. In addition to chromosomal, mendelian, and nonmendelian genetic conditions reviewed in this chapter, medical genetics includes prenatal and preimplantation genetic diagnosis, as well as newborn genetic screening, which are discussed in Chapters 14 and 32, respectively.
Genetic disease is common. Between 2 and 3 percent of newborns have a recognized structural defect. In another 3 percent of individuals, a defect is diagnosed by age 5, and another 8 to 10 percent of persons are discovered by age 18 to have one or more functional or developmental abnormalities. Advances in genomics are used increasingly to provide information regarding susceptibility to genetic diseases, and every indication suggests that this ield will reshape prenatal diagnosis.
Completed in 2003, the Human Genome Project identiied more than 25,000 human genes and led to rapid expansion of genomic research to better understand disease biology (McKusick, 2003). More than 99 percent of our DNA is identical. However, genetic code varies every 200 to 500 base pairs, usually as a single-nucleotide polymorphism. The human genome contains more than 80 million such genetic variants, and understanding their potential role in disease requires not only sophisticated interpretation but also integration of resources (Rehm, 2015).
The National Center for Biotechnology Information (NCB!) maintains genetic and genomic databases that are freely accessible to clinicians and researchers. Several of these databases are particularly useful in obstetrics and maternal-fetal medicine practice. he GeneReviews database provides in-depth clinical information for nearly 700 genetic conditions, including diagnostic criteria, management, and genetic counseling considerations (National Center for Biotechnology Information, 2017a). The Genetic Testing Registry (GTR) database contains information regarding the beneits and limitations of available tests for a given disorder. It lists more than 48,000 genetic tests and instructions for specimen collection and transport to individual laboratories throughout the world (National Center for Biotechnology Information, 2017b). Another database, Online Mendelian Inheritance in Man (OMIM), is a comprehensive catalog of human genes and phenotypes that allows clinicians to search for syndromes based on particular traits or abnormalities. As of early 2017, OMIM included more than 15,000 genes and nearly 5000 mendelian and mitochondrial conditions with a known molecular basis Oohns Hopkins University, 2017). The National Library of Medicine (2017) has also established a database of genetic information intended for patients-one that trainees may find especially helpful-the Genetics Home Reerence (GHR). his database contains data on more than 2400 genetic conditions and genes, including resources for families.