In facioscapulohumeral muscular dystrophy (FSHD), a small section of the DNA on chromosome 4 that’s shorter than usual is inherited in an autosomal dominant pattern, meaning it only takes one such mutation (from one parent) to cause the disorder. This altered piece of DNA also can occur spontaneously in a child as he or she develops in the womb.
FSHD can affect either males or females. In a small number of people with FSHD, the usual chromosome 4 mutation can’t be identified. In most affected people, it can be, with genetic testing. (For more on this, see Diagnosis .)
FSHD is one of many genetic disorders in which germ line mosaicism  is believed to occur. Germ line refers to egg or sperm cells. In this phenomenon, some sperm or egg cells in a parent carry a particular mutation.
In families with more than one child with FSHD but no previous family history, it’s likely that one parent has germ line mosaicism and that affected children were conceived with egg or sperm cells carrying the FSHD mutation. In these situations, the parents have no symptoms, and, if their blood cells are tested, they don’t show the mutation.
For more about genetics of FSHD and other neuromuscular diseases, see Facts About Genetics and Neuromuscular Diseases .
For help in understanding your family’s specific situation and planning for future children, it’s best to meet with a genetic counselor. You can obtain a referral to a counselor through your MDA clinic .