Becker muscular dystrophy (BMD) is one of nine major forms of muscular dystrophy, a disease characterized by progressive muscle weakness and atrophy that predominantly affects boys.

BMD shares many of the same symptoms with Duchenne muscular dystrophy (DMD), although they tend to be milder and progress more slowly. This is because both DMD and BMD are caused by various mutations in the DMD gene.

Inheritance of BMD

Genes are organized in chromosomes. We all have two copies of each chromosome, one inherited from our mother and one inherited from our father. However, the final pair, or the “sex chromosomes,” are not the same. While females inherit two X chromosomes (one from their mother and one from their father), males inherit an X chromosome from their mother and a Y chromosome from their father. These two chromosomes carry different genes.

BMD is an “X-linked disease,” meaning that the disease-causing gene is located on the X chromosome. This results in the condition mostly affecting males, as they only have one copy of the X chromosome. Females have an extra copy of the X chromosome, usually carrying a healthy DMD gene, and this healthy gene can compensate for the faulty one.

A man with BMD will always pass the mutation on to his daughters, but never to his sons as his sons inherit the Y chromosome from their father.

The condition is far less likely to manifest in females, since inheriting two mutated copies of the DMD gene is usually necessary for a person to develop BMD. If a girl inherits one mutated copy of the DMD gene, she will be known as a “carrier” and be at risk of passing the mutation to her children, possibly without knowing that she was carrying a faulty gene or showing any symptoms of BMD.

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