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page 1 | page 2 | page 3 | page 4 | page 5 | page 6 | page 7 | page 8 | page 9 | page 10 | page 11 | table of contents | references | test Historical Review of Genetics Through the ages, much speculation has occurred about heredity or how physical and functional characteristics were passed down through families. Philosophers and scientists conjured up theories of how transmission of traits to offspring occurred. Developments in technology and research capabilities have lead to accelerated progress of the understanding of the science of genetics. This progress is now enhancing the application of genetics in health care. History Early theories about how traits were carried (by the blood), contained (in the ovaries), or transmitted through families (skills absorbed over time passed onto children) were found to be inaccurate as microscopes and other tools were invented. The 18th century brought even greater examples of the role of selective reproduction in improved agricultural products which initiated a whole scientific field focused on heritable characteristics (Lander & Weinberg, 2000). In 1865, Gregor Mendel wrote a paper that described discrete hereditary elements in the sex cells as responsible for the transmission of traits. This work laid the foundation for modern genetics, although this was not realized until the late 1900s (Mendel,1865/1966). Traits that are passed on following the principles of inheritance described by Mendel are known as single-gene or Mendelian disorders (Thompson, Mclnnes, & Willard, 1991). More than 10,000 pieces of information about single-gene disorders has been identified and catalogued on the Internet in Victor A. McKusick's Mendelian Inheritance in Man site [http://www3.ncbi.nlm.nih.gov/Omim]. Throughout the 1900s, discoveries further identified the location of hereditary instructions in the genes found in chromosomes, recognized the role of genes in the production of enzymes, and elucidated the connection between a specific gene and its product (Muller, 1951). The molecular structure of DNA (deoxyribonulcleic acid) was determined by James Watson and Francis Crick (1953). The structure of DNA or the double helix makes gene transmission possible through the ability to make exact copies of itself. These strands of DNA are packaged in chromosomes in the cell’s nucleus and occur in pairs. DNA contains four purine-pyrimidine bases, adenine (A), guanine (G), cytosine (C), and thymine (T). A genetic code, made up of a series of three nucleotides (A, T,C, or G) guides the assembly of proteins. This precise genetic code is read out within the human body and specifies the organisms shape and function. RNA (ribonucleic acid), which contains uracil instead of thymine, carries this code to protein-making sites in the cell. Any error in the code affects protein synthesis. Such an error or mutation may be insignificant, may lead to disease, or may be lethal (Lewin, 2000). DNA technologies have been extraordinarily successful in unraveling the genetic code of multiple organisms including the fruit fly, E. coli, yeast, and now humans. The intricacies of the genetic code hold the key to improved understanding of human genetic disorders in the 21st century as a result of enhanced research methodology and funding.
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