The major site of motor neuron degeneration classifies these disorders. Common motor neuron diseases (MNDs) include amyotrophic lateral sclerosis, which affects both upper and lower motor neurons. Progressive bulbar palsy affects the lower motor neurons of the brain stem, causing slurred speech and difficulty in chewing and swallowing. Individuals with these disorders almost always have abnormal signs in the arms and legs. Primary lateral sclerosis is a disease of the upper motor neurons, while progressive muscular atrophy affects only lower motor neurons in the spinal cord.
Inherited MNDs are classified according to the mode of inheritance. Autosomal dominant means that a person needs to inherit only one copy of the defective gene from one affected parent to be at risk for the disease. There is a 50 percent chance that each child of an affected person will be affected. Autosomal recessive means that the individual must inherit a copy of the defective gene from both parents to be at risk for the disease. In this case, the parents are likely to be asymptomatic (presenting no symptoms of the disease). Autosomal recessive diseases often affect more than one person in the same generation (siblings or cousins). In X-linked inheritance, the mother carries the defective gene on one of her X chromosomes and passes the disorder along to her sons. (Males inherit an X chromosome from their mother and a Y chromosome from their father, while females inherit an X chromosome from each parent. Daughters have a 50 percent chance of inheriting their mother's faulty X chromosome and a safe X chromosome from their father, which would make them asymptomatic carriers of the mutation).This answer is based on source information from the National Institute of Neurological Disorders and Stroke.