Muscular dystrophy - Causes of muscular dystrophy

In most cases, muscular dystrophy (MD) runs in families. Usually, you develop it after inheriting a faulty gene from one or both of your parents.

MD is caused by mutations (alterations) in the genes responsible for healthy muscle structure and function. The mutations mean that the cells that should maintain your muscles can no longer fulfil this role, leading to muscle weakness and progressive disability.

Inheriting muscular dystrophy

You inherit your genes from your parents, with one copy of each gene being inherited from each parent (see box, right). If one or both of your parents has a mutated gene that causes MD, it can be passed on to you.

Depending on the specific type of MD, the condition can be a:

• recessive inherited disorder
• dominant inherited disorder
• sex-linked (X-linked) disorder

In a few cases, the genetic change (mutation) causing MD can also develop as a new event in the family. This is known as a spontaneous mutation.

These are explained in more detail below.

A recessive inherited disorder

If you have a recessive inherited disorder, it means you have inherited an altered version of the gene that causes the condition from both of your parents (both your copies of the gene are altered).

If a child only inherits an altered version of the gene from one parent, they will become a carrier of the condition. This means they are not affected, but there is a chance any children they have will be if their partner is also a carrier.

If both parents carry an altered version of the gene that causes the condition, there is a:

• one in four chance their child will have MD
• one in two chance their child will be a carrier
• one in four chance their child will be healthy (not inherit any mutated genes)

Some types of limb-girdle MD are inherited in this way.

A dominant inherited disorder

A dominant inherited disorder means you only need to inherit the mutated gene from one parent to be affected.

This means that if you have a child with an unaffected partner, there is still a 50% chance of your child developing the condition.

Types of MD inherited in this way include myotonic dystrophy, facioscapulohumeral MD, oculopharyngeal MD and some types of limb-girdle MD.

A sex-linked (X-linked) disorder

Chromosomes are long, threadlike structures that contain DNA and are found in each of the body's cells. Most chromosomes are the same in males and females, except for the pair of sex chromosomes, the X and Y chromosomes. A male has one X and one Y chromosome, whereas a female has two X chromosomes.

A sex-linked disorder is caused by a mutation in a gene on the X chromosome. As males only have one copy of each gene on the X chromosome, they will be affected if one of those genes is mutated.

As females have two copies of the X chromosome, they are less likely to develop an X-linked condition because the normal copy of the chromosome can usually compensate for the altered version. 

Females can still be affected by X-linked disorders, but the condition is usually less severe than when the gene alteration is present in an affected male.

Types of MD inherited this way include Duchenne MD and Becker MD, which is why these conditions are more common and more severe in males.

Spontaneous gene mutations

Occasionally, spontaneous gene mutations can cause MD. This is where the genes mutate for no apparent reason, changing the way that the cells function. Spontaneous gene mutations can cause MD to develop in people who do not have a family history of the condition.

Another way a child with no family history can be affected is when the condition is recessive. The gene mutations may have been present on both sides of the family for many generations, but may not have affected anyone until a child inherited a copy of the altered gene from both parents.