Please note: This is a basic revision aid, not a comprehensive and reliable statement of medical fact. These notes should not be used to guide treatment or diagnosis. For reliable information and advice, consult a qualified practitioner.
Many conditions have some form of genetic influence. In multiple sclerosis, for example, first degree relatives are estimated to have a 5-15% risk, compared to 0.2% for the general population. In Alzheimer's type dementia, the average risk at age 60 is <1%, but for first degree relatives it is estimated to be three times the average risk for that age. Parkinson's disease carries a 0.5% risk for the general population at age 70, but for first degree relatives it is about 1.5% (higher if the affected person had earlier onset of the disease). In all of these diseases, heredity is only one, perhaps amongst many, influences. The genetic influence may consist of an inherited increased susceptibility to environmental or other influences, or it may be that more than one gene defect is required to cause the condition.
There are also diseases where the majority of cases are sporadic, but in a clear minority of cases there is an increased familial risk. Motor Neurone Disease is an example of this.
Chromosomal abnormalities have been found to be the cause of some diseases, such as Down's Syndrome. In Down's Syndrome, there are three copies of chromosome 21, where there should ordinarily be two. This characteristic is passed to the child by the ovum, or sometimes the sperm cell, so the extra chromosome is present from conception. This appears to be a random mutation with a greater risk as the parent ages. The risk if the woman is under 20 is 1:2000. If the mother is over 45, or the father over 55, the risk is around 1:50.
Mitochondrial diseases are disorders of the mitochondrial protein encoded in the nuclear DNA. They are passed on by only the mother, as sperm carries no mitochondria. Children of either sex can be affected.
If a person has a genetically influenced disease, they should receive genetic counselling, from a specialised clinical geneticist, as the implications of the diagnosis for the client and for other family members can be devastating. There is also the question of whether to have a test if there are no symptoms but there is a family history of the disease. There is a predictive test for Huntington's Disease, for example, and the asymptomatic child of a person with the disease is faced with a very difficult choice: is it better to know or not to know? A positive test may also have implications for insurance and employment.
A diagnosis of a genetic condition obviously means the person will have difficult choices to make when deciding whether to have children, and in some cases both partners would need to be tested to determine risk.
Some diseases have a much clearer genetic influence that follows a fixed pattern, and are described below.
In Autosomal Dominant Inheritance, if one parent of either sex is affected, there is a 50% risk of each child being affected. There is no such thing as a carrier, though in some conditions, the person will not know that they have the gene until they have already had children.
Each of the five diagrams below represents one of four different average patterns of inheritance:
X-Linked Recessive diseases almost always affect males, with females being carriers. If one parent (the mother) is a carrier, each son has a 50% risk of being affected and each daughter has a 50% risk of being a carrier. If the father is affected, each daughter will be a carrier, but sons will not carry the gene.
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Last updated: 20th January 2003. Editor: Mike Griffin, OT (field of practice: community physical disability, UK). Any opinions expressed are the author's own unless otherwise stated. © Copyright OTdirect.co.uk 2002. All material is subject to copyright. Feel free to use it for any non-profit purposes, within your professional judgement, but commercial use is forbidden without permission.
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