Erin Kiernan, a teenager with Friedreich's Ataxia, monitors her cardiac function closely and gets help from several specialists. Introduction Questions and Answers Does It Run in the Family? MDA's Search for Treatments and Cures MDA Is Here to Help You

DOES IT RUN IN THE FAMILY ?

Two of this family's three children have FA.

On being told their child has a genetic disease like FA, bewildered parents often ask, "But it doesn’t run in the family, so how could it be genetic?"

The answer is that the mutations underlying FA can run silently through a family, because the disease is inherited in an autosomal recessive pattern.

Autosomal refers to the fact that the frataxin gene is on chromosome 9, one of the 22 pairs of autosomes (chromosomes other than the X or Y). Recessive means it takes two defective copies of the frataxin gene to cause FA, with one copy inherited from each parent, neither of whom would normally have FA symptoms.

Thus, FA might seem to occur "out of the blue." But in reality, both parents are FA carriers, each silently harboring a mutation in frataxin. Many parents have no idea that they’re carriers of FA until they have a child who has the disease.

About 1 in 100 Americans are FA carriers, but in some ethnic groups the frequency is higher. For example, about one in 70 people of Acadian (Cajun) ancestry are carriers.

The most common type of mutation in the frataxin gene is called a trinucleotide repeat expansion. Spelled out in the four chemical "letters" that make up DNA, it looks like a stutter in the frataxin gene.

Normally, the gene contains five to 30 repeats of the three-letter chemical phrase "GAA," but in people with FA, the gene can contain hundreds to thousands of GAA repeats. Longer repeat expansions tend to cause an earlier onset and faster progression, but the association isn’t strong enough to predict the course of FA in individual cases.

In more than 95 percent of people with FA, both copies of the frataxin gene contain expanded repeats. In the rest, just one copy of the frataxin gene is expanded and the other contains a single-letter change in the DNA code, called a point mutation.

In FA carriers, the frataxin gene can contain a repeat expansion, a point mutation or a premutation — a number of expanded repeats that’s just below the disease-causing range. In the germ line (ova and sperm), premutations might or might not expand into the disease-causing range, which makes it complicated for some carriers to determine their risks of passing on FA.

As a general rule, a child with a biological sibling affected by FA has a 25 percent chance of inheriting the disease.

Your MDA clinic physician or genetic counselor can give you more information about the risks of inheriting or passing on FA. Also, see MDA’s pamphlet, "Genetics and Neuromuscular Diseases."

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