In Duchenne and Becker muscular dystrophy cases, the most common situation is that the mother carries the faulty gene and passes it to her children, with only her male children showing symptoms of either condition. However, some women also show symptoms themselves that relate to the genetic fault that causes Duchenne muscular dystrophy or Becker muscular dystrophy , which makes them manifesting carriers. Until recently this condition has been little understood and infrequently diagnosed. It's now estimated that, among women known to be carriers of Duchenne muscular dystrophy or Becker muscular dystrophy, one in 10 might fall into this category.

Manifesting carriers' symptoms vary hugely - one girl may have a muscular dystrophy that is as severe as boy with Duchenne muscular dystrophy and another may suffer only mild problems with muscle weakness much, much later in life. However, common symptoms include aches and pains in the leg and arm muscles. This muscle weakness tends to progress with time – it is rare for there to be any sudden deterioration. Less commonly, manifesting carriers may also have problems with their heart muscle and intellectual capabilities.

Not all manifesting carriers have a family history of Duchenne or Becker muscular dystrophy. However, in the days before it was possible for laboratory staff to look at the dystrophin gene (the gene affected by both conditions) and protein a woman could only be suspected of being a manifesting carrier if she had a family history of the condition, or an affected son. Now that it’s possible to study muscle biopsy samples with dystrophin staining, the diagnosis has been made in women and girls who were previously thought to have a form of limb girdle muscular dystrophy.

It’s an important diagnosis. Unlike most of the forms of limb girdle muscular dystrophy, the risk of having a child with Duchenne or Becker muscular dystrophy is relatively high – like any carrier of either condition, a manifesting carrier has a one in four chance of having an affected son in any pregnancy. There is no evidence at present though to suggest that being a manifesting carrier tends to run in families.

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