There are about 100,000 different proteins whose manufacture is determined by the genetic material. A bit of the genetic material which is concerned with one protein is called a gene, so there are about 100,000 genes in the egg and a similar number in the sperm. The chemical structure of the DNA of a particular gene contains a code for the corresponding protein. Here is an example. Normally, humans have a gene which codes for a protein called dystrophin, essential for the integrity of muscle fibres.
Normal people have dystrophin in their muscle fibres.
Males with Duchenne type MD have something wrong with that gene. Their muscle fibres cannot make dystrophin properly and their muscles cannot function normally and suffer progressive damage. By definition, all MD are genetic, but they are not always inherited. In other words there is not always a history of MD in a previous generation of an affected person's family for a number of reasons. First a person may be affected by a genetic defect which has just occurred in a family for the first time, in one egg or one sperm of his or her parent.