What Causes Becker Muscular Dystrophy?
BMD occurs when a gene on the X chromosome changes (mutates) and fails to produce enough of the muscle-protecting protein dystrophin.
Because of the way BMD gets passed down from parent to child, it affects young boys. Every boy inherits an X chromosome from his mother and a Y chromosome from his father. The gene that can carry a change in dystrophin production is on the X chromosome.
BMD usually doesn't affect girls because girls inherit two X chromosomes (one from each parent). When a girl inherits a mutated dystrophin gene from one parent, she also receives a healthy one from her other parent, protecting her from the disease. Because boys don't receive a second dystrophin-producing gene to balance out the mutated one, they are affected by BMD.
Female carriers typically don’t have any symptoms of BMD, however up to 20 percent may show symptoms of cardiomyopathy or other muscle weakness. Sons of women who carry a mutated gene have a 50 percent chance of having BMD. Daughters of female carriers have a 50 percent chance of being BMD carriers.
BMD usually runs in families, but it's possible for a family with no history of the condition to have a son who has the disease or a daughter who’s a carrier. Sometimes new dystrophin gene mutations occur in a mother's egg cells. Once a new mutation passes to a son or daughter, it can be passed to the next generation.