This is an autosomal dominant (* chromosome 19: q13.2-13.3, cloned: Science 255: 1253 & 1256, 1992; protein is "myotonin" Science 260: 235, 1993), variably expressed because of other genes, affecting several systems. In some communities, it is as common as Duchenne's.
weakness, starting in the facial muscles (tiny chins and temples, etc.), eventually atrophy of most muscle groups
grip myotonia, with difficulty letting go of keys, handshakes, etc.
percussion myotonia (muscles tighten when you rub then)
frontal baldness and testicular atrophy (men)
a distinctive face ("carp mouth")
early dementia (sometimes)