A genetic disorder is a relatively rare genetic problem caused by one or more defects in an individual's DNA. The size and severity of these abnormalities can vary, ranging from a point mutation that changes a single base in a gene to involving entire chromosomes. Some of these defects can be caused by new mutations in the DNA, for example in Down syndrome. This is caused by a condition called trisomy 21, whereby the individual has a third copy of chromosome 21. However, the affected individual's parents are usually unaffected and the extra chromosome occurs by chance (1). Other genetic diseases can be hereditary, whereby faulty genes are inherited from the individual's parents. There are many well-known examples of inherited genetic diseases, such as sickle cell anemia, cystic fibrosis, and muscular dystrophy. This essay will focus on an inherited genetic disorder known as Huntington's disease, detailing the biology of this condition and its inheritance pattern along with how the disease is diagnosed and treated. Say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get an original essay Huntington's disease is an autosomal dominant neurodegenerative disorder that affects approximately 5-10 people per 100,000 in the European population, with a much lower prevalence in Asian countries and African communities (2). The disease is characterized by unpredictable and involuntary body movements, known as chorea, along with cognitive decline and behavioral differences (3). Typically, symptoms begin to appear between the ages of 30 and 50, however, a small portion of individuals may develop the disease before the age of 20 in a condition known as juvenile Huntington's disease (4). Huntington's disease is typically fatal within 15 to 20 years of onset, however resulting complications such as pneumonia, heart disease, or physical injury can shorten life expectancy even further (5). Furthermore, psychiatric and behavioral changes can result in depression, and between 8 and 9% of deaths in patients with Huntington's disease are attributed to suicide (6). The gene implicated in Huntington's disease is known as the Huntingtin (HTT) gene, which has been mapped to the short arm of chromosome 4p16.3 (7). The genetic mutation that occurs in this gene is known as a trinucleotide repeat, specifically of the CAG codon. The CAG repeat occurs in the coding region of the gene and codes for an amino acid glutamine that is included in the protein. In a normal huntingtin gene there are approximately 10-35 CAG repeats, which lead to the production of a normal protein. Furthermore, the inclusion of glutamines due to multiple CAG repeats causes a malformed protein and for this reason Huntington's disease is sometimes referred to as "polyglutamine" disease (7). A number between 27 and 35 repeats is known as an "intermediate allele", meaning that the individuals themselves are unaffected but the potential exists for affected offspring through further expansion of the CAG repeats. 36-39 repeats result in an allele with reduced penetrance, so symptoms usually appear at an older age. Therefore, individuals with fully penetrant Huntington's disease typically have 40 or more CAG repeats (8). The presence of CAG repeats also leads to a phenomenon known as “anticipation”, whereby the greater the number of CAG repeats, the earlier the onset of the disease (3). Therefore, in individuals with juvenile Huntington's disease, the number of CAG repeats often exceeds 55 (9). It is not entirely clear why the mutated Huntingtin protein is toxic to cells.