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The Human Genome Project: What It Means for You
Genetics is becoming a factor in the diagnosis and treatment of all kinds of disorders. While aware of medical genetics' growing importance, many people, both doctors and patients, feel it is difficult to keep pace with this rapidly advancing area. Famed geneticist, Dr. Jim Lupski, outlines the basic terrain of the genome project and introduces some of its issues and vocabulary.
— The Editors
Birth defects are the leading cause of death among infants (0-364 days old). By age 25, eight percent of liveborns will be diagnosed with a disorder that has a major genetic component. The goals of the human genome project are to identify all genes in the human genome, determine their structure and function and determine their involvement in human disease.(8) Let's see if we can walk through some of the basic concepts and terminology.
Mutation and Human Genetic DiseaseAs mentioned, 8% of liveborns will suffer from a genetically based disorder by age 25.(1) We refer to the all these genetic defects as the genetic load. Genetic load is produced by five paths:
Genome MutationsThe genome is defined as the total genetic material contained within the chromosomes of an organism. Humans have 46 chromosomes, 23 from each parent. Our 46 chromosomes represent our genome.
Genome mutations occur at a frequency of about 1/10,000 to 1/100,000. Polyploidy occurs when an extra copy of every chromosome is made, trisomy when only an extra copy of a single chromosome occurs. Trisomy 21 (Down syndrome) is perhaps the most frequently observed genetic disease, occurring in 1:660 newborns. If an entire chromosome is absent, most frequently in Turner syndrome, the disorder is called a monosomy.
Chromosome MutationsStructural rearrangements in chromosomes that can be detected microscopically by changes in the banding patterns produced by dye specific staining of chromosomes are said to be chromosomal mutations. There are several major types:
Microdeletionsinvolve deletion of a small portion of a chromosome, sometimes several contiguous genes, produce a set of syndromes which are now being appreciated to a much greater extent.
Microduplicationsproduce an extra exact copy of a specific region of a specific chromosome.
Inversionsre-order a segment of chromosome in backwards order.
Translocationsoccur when a piece of a chromosome attaches to another chromosome. Individuals who harbor these translocations are said to be balanced (because they have the proper amount of genetic information) but their offspring can, as the result of incorrect attachment producing duplications and deletions, be unbalanced. It is estimated that one in every 550 individuals on this earth may contain a balanced translocation. If three or more miscarriages occur in a family, unbalance should be suspected and the family's chromosomes investigated.
Gene MutationsGenes are the most basic unit of heredity and are built from DNA. The DNA molecule is constructed from two vertical strands of chemicals (bases) — adenine (A), guanine (G), cytosine (C) and thymine (T) — coiled around each other like a twisted ladder (the double helix). Each parent contributes one of the vertical strands. The DNA ladder is held together by rungs formed by bonds between bases (base pairs). Thousands of base pairs are needed for each gene. It is believed that the human genome has between 50,000 and 100,000 genes. Gene mutations occur in a frequency believed to be between 1/100,000 to 1/1,000,000.
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