FSHD is a dominant hereditary disease. This means that the inheritance of a defective copy of a DNA segment from a parent with FSHD is sufficient to cause disease. The genetic defect in FSHD occurs at one end of chromosome number 4. All individuals carry two copies of their DNA in each cell, but pass on one copy to their offspring. A parent with FSHD FSHD will bear the genetic defects in one of two copies of chromosome 4, as shown in red in the diagram below. Therefore, an affected parent has a chance of 50: 50 transmission of the defective copy of their child as shown in the diagram below.
There are however, exceptions to this pattern of inheritance. Up to 30% of people with FSHD, no signs of FSHD is detected in parents and tests show no genetic defect on chromosome 4. Such a situation, called sporadic FSHD, means that the genetic defect in FSHD occurred spontaneously in this individual. These individuals may, in turn, pass this defect to their offspring.