FSHD is caused by certain gene changes (mutations). A gene called DUX4 is normally inactive in most cells in the body but gets activated in FSHD. Other genetic factors play a role in FSHD type-2, which is less common. Both types cause similar problems.
FSHD affects both boys and girls. Either parent can pass it down to their children. This is an autosomal dominant inheritance pattern. A parent with the FSHD gene has a 1 in 2 chance of passing it on to each of their children.
Sometimes a child may have FSHD even though a parent doesn't have the condition. This may happen because the defect that causes FSHD is found in some of the parent's sperm or egg cells but not in the rest of the parent's body cells. In this case, the parent may have an increased risk of passing the defect on to any of their other children as well.
In some cases, FSHD results from new mutations instead of through inheritance.