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 how genes function and interact. In addition to chromosomal, mendelian, and nonmendelian genetic conditions reviewed in this chapter, medical genetics includes preimplantation and prenatal diagnosis, gene therapy, and newborn screening, which are discussed in Chapters 14, 16, 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 are discovered by age 18 to have one or more functional or developmental abnormalities. An astonishing two thirds of the population will experience a disease with a genetic component during their lifetime. Advances in genomics are used increasingly to provide information regarding susceptibility to genetic diseases, and there is every indication that this field will reshape prenatal diagnosis (Bodurtha, 2012).
Completed in 2003, the Human Genome Project identified nearly 25,000 human genes and led to rapid expansion of genomic research to better understand disease biology (Bodurtha, 2012; Feero, 2010; McKusick, 2003). One example is the International HapMap (Haplotype Map) Project, which studies the effects of genetic variation (National Human Genome Research Institute, 2012). HapMap investigates the nearly 10 million single nucleotide polymorphisms that comprise 0.5 percent of our DNA. Researchers look at how groups of common polymorphisms affect factors such as propensity to particular diseases and response to treatment. Another example is dbGaP, the database of Genotypes and Phenotypes, which is maintained by the National Center for Biotechnology Information (NCBI) (2013a). This database includes studies of genotype and phenotype interactions, such as genome-wide association studies and medical diagnostic assays. It is hoped that data from dbGaP will be used to develop tests or products that address public health needs.
The NCBI also maintains several genetic and genomic databases useful in obstetrics and maternal-fetal medicine practice. These include Online Mendelian Inheritance in Man (OMIM), GeneTests, and the Genetics Home Reference. Each is freely accessible to clinicians and researchers. Of these, OMIM is a comprehensive catalog of human genes and phenotypes originally created by the National Library of Medicine in collaboration with Johns Hopkins University. Clinicians can use OMIM to gain detailed information regarding particular syndromes and their genetic basis. Or, if a syndrome is suspected but the diagnosis is unclear, it may aid formulation a differential diagnosis by searching for syndromes that include particular traits or abnormalities. As of 2013, OMIM included more than 14,000 different genes with known sequences and nearly 4000 mendelian or mitochondrial conditions—phenotypes—with a ...