In around two thirds of the cases of Duchenne the genetic change is passed down through the family from the mother.

Each person inherits a set of genes from their father and another set from their mother. The genes have been copied from the parents' cells into the child's cells.

Genes are found on 'chromosomes'. The DMD gene is located on a chromosome called the X chromosome. Boys have one X chromosome and one Y chromosome; girls have two X chromosomes.

DMD is inherited in a pattern called 'X-linked inheritance'. The DMD gene is 'carried' by women, but does not usually cause problems in girls or women (with rare exceptions, below). This is because of there being two X chromosomes in women: one X chromosome has the 'faulty' DMD gene, and the other X chromosome has a normal gene, which compensates for the faulty one.

In contrast, boys with the DMD gene do not have a second X chromosome and so they cannot compensate for the faulty gene. Therefore, boys with the DMD gene always have symptoms of the disease.

Overall with each pregnancy, the likelihood of each of the 4 outcomes is equal. There is a 1 in 4 chance that a boy with DMD will be born. There is also a 1 in 4 chance that a daughter who carries the DMD gene will be born.

On the other hand, if a man with Duchenne was to have children with a non-carrier woman, none of their sons would have Duchenne although all of their daughters would be carriers.