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The principles of genetics are relatively simple. However, the complexity of the 3 billion nucleotides in the human genome means that these simple principles must be applied to a remarkably variable information base. This book emphasizes the structure, organization, and physiologic consequences of genetic variations in humans. Sequencing the human genome has identified an unanticipated range of variations; newer techniques likely will find many more. Thus, how the basic principles will be translated from this variant base of DNA through cellular metabolism and physiology cannot currently be predicted. The high frequencies of single nucleotide polymorphisms, copy number variations, inversions, deletions, amplifications, and epigenetic changes already discovered have no precedents—-fully integrating their consequences likely will be complicated. All of this means that applying genetics to human and medical biology will remain a challenge.
In the past, medical genetics often has been viewed as an obscure collection of observations about rare anomalies. Now, the striking variations found in sequence data mean that any aspect of medicine will require awareness of fundamental biologic differences, eg, in disease pathogenesis, natural history, reactions to the environment and drugs, and neoplasia. Large amounts of sequence information will soon become available for individual patients; how we use this will be related to our understanding of basic mechanisms and their interactions. No longer will genetics be limited to quaint, arcane rarities; it will have become part of the medical mainstream. I invite readers to embark on a fascinating journey.
Consistent with the plan of the Road Map series, this book emphasizes fundamental principles. No attempt has been made to be encyclopedic. Instead, specific disorders are presented as examples of these principles. Some of these disorders (eg, Down syndrome, phenylketonuria, sickle cell disease, neurofibromatosis, G6PD deficiency) appear in multiple contexts, emphasizing their relative frequency. Although these are quite illustrative, many others could have been chosen and the basic notions can be applied broadly.
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