Many neuromuscular diseases are genetic diseases, meaning they're caused by defective genes, and can be passed from one generation to the next. Understanding the facts about a genetic disease is important for family planning and for peace of mind. But it's a challenging task, made more difficult by common misconceptions about genetic diseases.
Since genetic diseases can be inherited, they often do "run in the family." But many people with genetic diseases have no family history of them.
This is especially true with recessive diseases, in which one or both parents might be carriers who silently harbor a single defective gene. For recessive disorders to "show themselves," it takes two defective genes — one from each parent — to come together in a child.
Genetic diseases can also occur "out of the blue" when a new mutation occurs during a child's conception or very soon afterwards. These are called spontaneous mutations, and after they occur, they can be passed on to the next generation.
Many people believe that certain diseases or even harmless traits (like baldness or eye color) consistently skip a generation. The truth is, certain diseases may skip a generation, but there's no disease that does this by definition.
Recessive diseases are probably at the root of this myth. Imagine that a man has a recessive disease, caused by two defective copies of gene A, while his wife has two normal copies of gene A. All of the couple's children will inherit one defective gene A and one normal gene A, making them carriers who don't have the disease.
If any of the children starts a family with someone else who carries the disease, there's a good chance the disease will appear in the third generation. The disease has "skipped" the second generation and appeared in the third — but there's no guarantee that it will skip the fourth.
This misconception probably arises from the fact that the most common childhood muscular dystrophy, Duchenne MD, is X-linked. It's carried on the X chromosome, which boys inherit from their mothers. Most neuromuscular diseases, including many types of muscular dystrophy, aren't X-linked and therefore have no sex bias.
Disorders that aren't X-linked are called autosomal, because chromosomes other than the X and Y are called autosomes. Many neuromuscular genetic diseases are autosomal. (Few genes are on the Y chromosome, present only in males, and so far it hasn't been an important source of mutations that cause genetic disorders.)
X-linked diseases are more common and typically more severe in males, but they can affect females as well.
With only one X chromosome, males don't have an extra gene to take up the slack when one X-linked gene is defective. Although females have two X chromosomes (one from each parent), sometimes a normal copy of an X-linked gene isn't enough to compensate for a defective one.
Upon finding out there's a genetic disease in the family, many people naturally search for answers about how the disease got there. Often, pop science tells us that defects, or mutations, in genes are caused by exposures to toxic chemicals or radiation, or by malnutrition.
It's true that intense exposure to certain toxins or radiation can significantly damage DNA (the chemical that makes up genes). But such damage is unlikely to cause inheritable mutations; it's more likely to cause health problems restricted to the exposed person — unless the damage hits a developing fetus or occurs at the time of conception.
Although malnutrition can cause birth defects (which aren't passed on to future generations), there's no evidence that it can cause genetic disease.
Most genetic diseases simply arise from random errors that cells make while processing or copying DNA.
Nowadays, people often look to genetic testing as a gold standard for diagnosing genetic disease. Genetic tests can detect disease-causing mutations, and thus can be used to help diagnose a genetic disease, or to probe carrier status.
Unfortunately, genetic tests for many diseases aren't sensitive enough to find all of the possible disease-causing mutations. So, a negative test result doesn't necessarily mean the tested person doesn't have (or carry) the disease in question.
For more information about the genetic bases of neuromuscular diseases, see MDA's pamphlet "Genetics and Neuromuscular Diseases."