Myotonic dystrophy (DM) was one the first autosomal dominant disease discovered to be caused by a repeat expansion.
As it is dominant, if one parent is affected, there is a 50% chance that the future child will inherit the disease. However, the mutation may first appear in a generation, which is called a mutation “de novo”.
All this transcription of the repeat expansion accumulate and, as toxic RNAs, disrupt the normal function of up to twenty other genes, causing all the symptoms typic of Myotonic Dystrophy.
Although the two different types of myotonic dystrophy have common symptoms, they have basically two different origins. The two diseases (DM1 and DM2) are caused by distinct expansions located both in the regions of different genes.