In these conditions, the mutation occurs in a gene which is located on the X-chromosome. This genetic fault causes a complete lack of dystrophin in Duchenne muscular dystrophy, or the absence of a fully-functional dystrophin in Becker muscular dystrophy.
What is a manifesting carrier of Duchenne or Becker muscular dystrophy?
Girls have two X-chromosomes and boys have only one. Therefore, if boys have a mutation in the dystrophin gene they will be affected by Duchenne or Becker muscular dystrophy. Most commonly, if a female carries the mutation in the dystrophin gene on one of the two X-chromosomes, the second X-chromosome usually ‘protects’ her from developing the symptoms of the condition because the second X-chromosome produces a normal dystrophin protein.
These girls are referred to as ‘carriers’. However, some carriers do show symptoms of the condition, because the second X-chromosome does not produce the dystrophin protein as it should. These carriers are called ‘manifesting carriers’.
Each son of a carrier mother has a 50:50 chance of being affected and each daughter has a 50:50 chance of being a carrier herself, although she might be an asymptomatic – rather than a manifesting – carrier.
It is important that carriers, and other family members at risk, receive genetic counselling and advice with regard to testing future pregnancies. Your clinician or GP can arrange this for you.
It is very important to note that the diagnosis of ‘manifesting carrier’ may be made in someone who has absolutely no family history of Duchenne or Becker muscular dystrophy.
It is an important diagnosis to make in these cases because the risk of having a child who may be affected with Duchenne or Becker muscular dystrophy is relatively high.
All carriers of Duchenne or Becker muscular dystrophy – both asymptomatic and manifesting – have a one in two chance of having an affected son in any pregnancy. There is no clear evidence to suggest that being a manifesting carrier tends to run in families. So, if one carrier in any family has muscle problems, this does not seem to make it any more likely that other female members of the family will also manifest any symptoms relating to being a carrier.