Another common testing situation is carrier testing for autosomal recessive disorders. Autosomal recessive disorders are caused by the inheritance of two nonfunctioning genes, one from each carrier parent. The parents are referred to as carriers because they carry only one nonfunctioning gene and are, therefore, not affected by the disorder. Every individual is thought to be an unaffected carrier of some autosomal recessive disorder. This is only a problem, however, if two individuals who both carry the same recessive disorder conceive a child together. Under this circumstance, the child would have a one in four (25%) chance of inheriting a nonworking copy from each parent, thereby inheriting the disorder.
There are hundreds of genetic tests available, but it is not practical to perform every available test on each person. Carrier testing is typically offered only to those individuals who are at increased risk based on family history or ethnic background. While family history bears an obvious correlation, ethnic background is important because those who descend from the same group of ancestors are more likely to carry the same genetic changes. For example, individuals of Ashkenazi Jewish descent have about a one in thirty chance of being a carrier for a condition called Tay-Sachs disease,
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