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  • For many human diseases, the fundamental defect resides in a simple alteration in the genome—the master blueprint of DNA that orchestrates the basic operation of a cell and an organism. Genetic studies often provide the ability to define at a molecular level the nature of such DNA alterations (i.e., mutations). Knowledge of the normal and abnormal forms of genes is invaluable for understanding the basis of many human genetic diseases.

  • The haploid human genome consists of ≈3 billion base pairs (bp) of DNA that are distributed among 24 distinct chromosomes (22 autosomes and 2 sex chromosomes). Within this vast array of nucleotides are encoded an estimated 50,000 to 100,000 genes and the necessary elements that control the regulation of their expression.

  • Analyzing a genome involves the construction of various types of maps that reflect different features of the DNA, with the major classes being cytogenetic maps, genetic maps, and physical maps. The highest-resolution physical map is the DNA sequence map, which reflects the precise order of nucleotides along a chromosome. Important technological advances have produced a number of powerful methods that greatly facilitate the ability to analyze genomes.

  • The Human Genome Project (HGP) is a large, coordinated effort to elucidate the genetic architecture of the human genome and, in parallel, that of several model organisms. The initial phase of this endeavor has mostly involved constructing relatively low-resolution genomic maps and refining the approaches for large-scale DNA sequencing. The next phase of the HGP will focus more on establishing the complete nucleotide sequence of the human and other genomes as well as beginning to decipher the encoded information systematically.

  • The products of the HGP are providing a detailed working knowledge about the organization of human DNA and that of several model organisms as well as an infrastructure (in the form of biologic, informational, and technological tools) that is already ushering in a spectacular new era of biomedical inquiry. From a clinical viewpoint, this infrastructure is facilitating the identification and characterization of genes that directly and indirectly lead to human disease, which in turn should ultimately improve the ability to diagnose and treat affected individuals.


Diseases are associated with alterations of normal biologic processes and can be caused by infectious agents, environmental influences, genetic anomalies, or combinations of these factors. Human disease is classically studied by comparing affected tissues with their unaffected counterparts. Such studies often reveal biochemical and physiological differences, and this information can, in some cases, be used to formulate appropriate therapies. Though this approach has led to the development of a successful treatment for many diseases, it frequently fails to identify the fundamental etiology of the disorder itself. Indeed, the differences encountered in affected tissues are often due to secondary effects rather than consequences of the primary defect. However, in cases where DNA sequence alterations (i.e., mutations) are responsible for the disease, ...

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