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Thursday, July 24, 2014
Inherited Disorders and Birth Defects
1. Has there ever been a case where a patient has been diagnosed with Friedreich’s ataxia and only one parent was a carrier?
2. As the gene is passed down, is there a greater chance of being a carrier?
As you may know, Friedreich ataxia (FRDA) is a slowly progressive loss of balance and problems coordinating movement of legs and walking (called ataxia) that usually starts in the mid teens and usually before the age of 25 years. People with FRDA usually do not have reflexes in the legs, have trouble swallowing or problems with speech, may have a loss of sensation in the lower legs and muscle weakness. People with FRDA can also have other problems, such as heart problems or diabetes.
The disease by a mutation (change) in a gene called FXN that is on chromosome 9. It is inherited in an autosomal recessive manner. What means is that both parents are carriers (have one normal form of the gene and one form that causes the disease). If you only have 1 abnormal gene, you do not have the disease.
However, if both parents carry the gene that causes FRDA they have a 25% chance with each pregnancy of both passing the mutated gene on at the same time and having an affected child. They also have a 50% chance of having a child who is a carrier, just like they are. Finally, there is a 25% chance that any child they have will not inherit the gene that causes FRDA at all.
To answer your question - can only one parent be a carrier and still have a child with FRDA - usually no. It could be that there is non-paternity; the father of an affected person is not the father. It could be that the person does not really have FRDA, but another genetic disease which looks like Fredreich ataxia and is inherited differently (where only 1 parent needs to have the gene that causes the disease). Finally, one of the parents might have a different mutation (change in the gene) that was not looked for during genetic testing.
I believe your question about "as the gene is passed down through a family, is there a greater chance of being a carrier" has to do with the mechanism of how the gene for FRDA works (called a triple repeat), which is very complicated. In FRDA, if a person is a carrier for the disease, they have more of these triple repeats, but not enough to cause problems. When they have children, there is a chance that the number of these triple repeats can get bigger. However, that person's partner would also have to be a carrier for FRDA for a child to have the disease.
The genetics of Fredreich ataxia is quite complicated. If someone has specific questions, I would highly recommend that they speak to a genetic counselor or geneticist. A genetics center can be located by going to the National Society of Genetic Counselors Resource Center at the web site below.
Anne Matthews, RN, PhD
Associate Professor of Genetics
School of Medicine
Case Western Reserve University