In most cases, muscular dystrophy (MD) runs in families. It usually develops after inheriting a faulty gene from one or both 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 have two copies of every gene (with the exception of the sex chromosomes). You inherit one copy from one parent, and the other copy from the other parent. 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 mutation that causes 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've 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'll become a carrier of the condition. This means they're not affected, but there's a chance that 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's a:
- 1 in 4 chance their child will have MD
- 1 in 4 chance their child will be healthy, but carry the mother's faulty gene
- 1 in 4 chance their child will be healthy, but carry the father's faulty gene
- 1 in 4 chance their child will be healthy (won't 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's 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 of DNA. A male has one X and one Y sex chromosome, and 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'll be affected if one of those genes is mutated.
As females have two copies of the X chromosome, they're less likely to develop an X-linked condition, because the normal copy of the chromosome can usually cover for (mask) 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 in this way include Duchenne MD and Becker MD, which is why these conditions are more common and more severe in males.
Spontaneous gene mutations
Spontaneous gene mutations can occasionally cause MD. This is where the genes mutate for no apparent reason, changing the way the cells function. Spontaneous gene mutations can cause MD to develop in people who don't 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.