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INTRODUCTION

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Human genetics is the scientific study of the variations among people that are determined by the heritable units called genes, and how variations due to genes occur and are transmitted in individuals, families, and populations. While the discipline of genetics had its origins with Gregor Mendel in the mid-nineteenth century, human genetics began in the early twentieth century and remains one of the most vibrant of the biologic sciences. Genomics, on the other hand, had its origins much more recently, the term first being coined in 1986, and subsumes the study of the organization, function, and interpretation of all of an organism's genetic material. Genomics has largely been stimulated and driven by technologies that enable the sequencing of DNA and the comparative analysis of vast amounts of sequence data. Both fields are transforming our understanding of human biology, medicine, and public health.

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Clinicians at one time concerned themselves only with what they could discover by bedside evaluation and laboratory investigation. In the parlance of genetics, the patient's symptoms and signs constitute his or her phenotype. Now the means are at hand for defining a person's genotype, the actual information content inscribed in the 2 m of coiled DNA present in each cell of the body—or half that amount in every mature ovum or sperm. Most phenotypic characteristics—and this includes diseases as well as human traits such as personality, adult height, and intelligence—are to some extent determined by the genes. The importance of the genetic contribution varies widely among human phenotypes, and methods are only now being developed to identify the genes involved in complex traits and most common diseases. Moreover, the importance of interactions between environment and genotype in producing phenotypes cannot be overstated despite the obscurity of the actual mechanisms, such as the contributions of the epigenome and the microbiome.

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DNA is composed of four nucleotides—adenine (A), guanine (G), cytodine (C) and thymidine (T)—arranged linearly along one strand, which intertwines in a double helix with a complementary strand such that each A pairs with a T and each G with a C. Each human cell nucleus contains 6.4 billion of these nucleotide pairs. About 2% of nuclear DNA is organized in functional units called genes, and each of the approximately 23,000 human genes is accompanied by various regulatory elements that control when it is active in producing messenger RNA (mRNA) by a process called transcription. In most situations, mRNA is transported from the nucleus to the cytoplasm, where its genetic information is translated into proteins, which perform the functions that ultimately determine phenotype. For example, proteins serve as enzymes that facilitate metabolism and cell synthesis; as DNA binding elements that regulate transcription of other genes; as structural elements of cells and the extracellular matrix; and as receptor molecules for intracellular and intercellular communication. DNA also encodes many small RNA molecules that serve functions still being defined, including regulating gene transcription and interfering with ...

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