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Genetics is the study of genes, heredity, and the variation of inherited characteristics. Medical genetics addresses the etiology and pathogenesis of human diseases that are at least partially genetic in origin, as well as their prediction and prevention. Whereas a gene is a specific sequence of deoxyribonucleic acid (DNA) on a single chromosome that codes for a particular protein, a genome is the entirety of all genes that make up an organism. Genomics is the study of how genes function and interact with one other. Chromosomal, mendelian, and nonmendelian genetic conditions are reviewed in this chapter. Prenatal and preimplantation genetic testing and newborn genetic screening are discussed in Chapters 17 and 32, respectively.

Genetic disease is common. Two to 3 percent of newborns have a recognized structural abnormality, another 3 percent are diagnosed with an abnormality by age 5 years, and yet another 8 to 10 percent are found to have a functional or developmental abnormality before reaching adulthood.


The Human Genome Project was an international research program that sequenced the 3 billion base pairs and more than 20,000 genes that make up the human genome. It paved the way for research into gene organization and function in an effort to understand the molecular basis of disease. More than 99 percent of our DNA is identical. The coding regions of DNA—exons—constitute only 1.5 percent of the genome. An exome is the entirety of all the exons that an organism contains. Introns are DNA sequences involved in coding regulation and make up 24 percent of the genome. Intergenic DNA composes the remainder.

Our genetic code varies once every 200 to 500 base pairs, usually as a single-nucleotide polymorphism (SNP). Thus, whole genome sequencing and whole exome sequencing, described later (p. 327), hold tremendous potential to further elucidate genetic variants in disease.

Toward this goal, genetic and genomic databases are maintained by the National Center for Biotechnology Information (2021). These are freely accessible and can be indispensable to providers who offer counseling and testing for genetic conditions. The GeneReviews database has in-depth clinical information for more than 800 genetic conditions, including diagnostic criteria and management considerations. The Genetic Testing Registry (GTR) database contains information for nearly 80,000 genetic tests and instructions for specimen collection and transport to individual laboratories throughout the world (National Center for Biotechnology Information, 2021). The National Library of Medicine (2021) has also established a genetic information database, the MedlinePlus Genetics, which is intended for the lay population. It provides explanations of more than 1300 genetic conditions, a glossary of genetic concepts, and links to more comprehensive medical resources.


Chromosomal abnormalities figure prominently in genetic disease. They account for >50 percent of first-trimester miscarriages, approximately 20 percent of second-trimester losses, and 6 to 8 percent of stillbirths and early-childhood deaths ...

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