Alexis Villa of California, born in 1995, has type 3 SMA. She uses leg braces most of the time, and occasionally a wheelchair for longer distances. She loves going to MDA summer camp and is seen regularly at an MDA clinic. Introduction Questions and Answers Does It Run in the Family? MDA's Search for Treatments and Cures MDA Is Here to Help You _________________ A Family Guide to the Consensus Statement for Standard of Care in Spinal Muscular Atrophy (PDF)

DOES IT RUN IN THE FAMILY ?

In this family, two of the six children have SMA because they each inherited a flawed gene from each parent. The other children may be SMA carriers, able to pass a flawed gene to their children.

When people learn that a family member has a genetic disorder, they often wonder how this could be the case if it doesn’t "run in the family."

The chromosome 5 type of SMA (the most common type) follows an inheritance pattern known as autosomal recessive, which often takes families by surprise. Diseases that are recessive require two gene flaws — usually one from each parent, but occasionally one from one parent and one that occurs as a fetus is being formed — before the disease shows itself. People who have only one gene flaw for a recessive disease are said to be carriers and usually show no symptoms. Often, a family has no idea that some members are carriers until a child is born with a recessive disorder.

The autosomes are the numbered chromosomes, that is, all the chromosomes except the X and the Y, which determine gender.

If both parents are carriers of SMN-related SMA, the risk of each pregnancy producing a child with the disease is 25 percent. This risk doesn’t change no matter how many children in the family have been affected previously. The "dice are rolled" with each new conception.

Genetic testing for chromosome 5 SMA is widely available for those suspected of having the disease, including unborn babies. However, as of 2003, carrier testing for SMA is more difficult and not widely available.

Genetic testing is expanding and changing rapidly, but its implications can be complex. It’s best to talk with a genetic counselor (you can obtain a referral through your MDA clinic) before embarking on testing.

SMA can occur even when there’s no family history.

In contrast to chromosome 5 (SMN-related) SMA, the gene flaw causing most cases of spinal-bulbar muscular atrophy (SBMA) is on the X chromosome, leading to an X-linked inheritance pattern.

Females have two X chromosomes, and males have an X and a Y chromosome. Females who have a gene flaw on one X chromosome are usually considered carriers of an X-linked disease (although sometimes they can have a mild form of the disease). Males, in contrast, have no second X to protect them from the full effects of a gene flaw on the X chromosome and show the full effects of such a flaw.

Males who inherit an X-linked gene flaw generally have the disease. Each son of a woman who carries an X-linked disease has a 50 percent chance of inheriting the gene flaw and developing the disease. Each daughter has a 50 percent chance of inheriting the gene flaw and being a carrier herself.

Genetic testing via a blood test is available for SBMA.

For more on genetics and genetic testing, see the MDA publication "Genetics and Neuromuscular Diseases."

Back to Disease Booklets