Motor neurone disease or ALS is characterised by a progressive muscle weakness and wasting, which affects movement of the limbs, speech, swallowing and respiration. In the majority of cases, this is not associated with intellectual impairment. Approximately 10-15 % of cases are thought to be inherited. The remainder are sporadic (affecting only one member of the family) and the cause is unknown. A number of forms of inherited or familial motor neurone disease have been identified. There may be incomplete penetrance, where not all individuals carrying a faulty gene causing motor neurone disease, will necessarily develop the condition. In some families where the condition is inherited, mutations have been identified in a gene called the SOD1, which is located on chromosome 21. In these cases, the inheritance of the faulty gene follows a pattern of inheritance called autosomal dominant inheritance. Other causative genes include VAPB and ALS2. Progressive bulbar palsy has an autosomal recessive pattern of inheritance. Werdnig-Hoffmann disease (spinal muscular atrophy type I) is transmitted as an autosomal recessive condition and the SMN1 and SMN2 genes causing this condition are located on chromosome 5. Kugelberg-Welander syndrome is an autosomal dominant condition. Benign focal amyotrophy and infantile spinal muscular atrophy are
inherited as autosomal dominant conditions.