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An abnormal gene on one of the autosomal chromosomes (one of the first 22 "non-sex" chromosomes) from each parent is required to cause the disease. People with only one abnormal gene in the gene pair are called carriers, but since the gene is recessive they do not exhibit the disease.
In other words, the normal gene of the pair can supply the function of the gene so that the abnormal gene is described as acting in a recessive manner. BOTH parents must be carriers in order for a child to have symptoms of the disease; a child who inherits the gene from one parent will be a carrier.
The inheritance of genetic diseases, abnormalities, or traits is described by both the type of chromosome on which the abnormal gene resides (autosomal or sex chromosome), and by whether the gene itself is dominant or recessive. This is due to whether a single defective gene from one parent (dominant inheritance) or both copies of the gene (one from each parent) are defective (recessive inheritance).
For an autosomal recessive disorder: When both parents are carriers of an autosomal recessive trait, there is a 25% chance of a child inheriting abnormal genes from both parents, and therefore of developing the disease. There is a 50% chance of each child inheriting one abnormal gene (being a carrier).
This does not mean that children WILL necessarily be affected. It does mean that EACH child has a one in four chance of inheriting the disorder and a 50:50 chance of being a carrier.
Update Date: 7/25/2003Douglas R. Stewart, M. D., Division of Medical Genetics, Hospital of the University of Pennsylvania, Philadelphia, PA. Review provided by VeriMed Healthcare Network.
Last updated: Tue, 06 Jan 2009 00:20:03 GMT