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| Picture 11. We have 23 pairs of chromosomes made from twisted DNA molecule. Each one has about 250 million bases. A gene is represented by a sequence of around 50 thousand bases. |
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| Monogenic diseases |
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The vast majority of gene-specific tests currently available and under development are for single gene (monogenic) disorders such as Huntington's disease and cystic fibrosis. These tests detect changes in a single gene, which lead to the disease. There are few, if any, effective treatments which can alter the progression of many of these diseases. This means that genetic testing for monogenic disorders has a number of ethical, legal and social implications. These include:
- possibility of discrimination against individuals with the disease, preventing them getting a job or some types of life insurance
- the psychological impact of the diagnosis on the patients and their families
- need for pre- and post-test genetic counselling.
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| Common complex diseases |
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Gene-specific tests for common complex diseases have the added complication that, although there may be changes in the genes, the change does not always cause the disease, but is a risk factor for the disease. The uncertainty about whether the person will go on to develop the disease, and not knowing how severe the disease will be, can make issues such as patient discrimination and genetic counselling more complex. |
| Huntington's disease |
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Huntington’s disease is a disease of the central nervous system. It develops in middle age and there is currently no cure. Sufferers gradually deteriorate until they require total care and eventually die.
The defective gene which causes this disease was identified in 1993, making genetic testing possible. People who believe they may have inherited Huntington's disease can be tested, but they are given extensive counselling before they decide whether or not to proceed, and afterwards if the tests are positive.
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