Neurodegenerative group

These conditions include:

• spinobulbar muscular atrophy (Kennedy's disease)
• Huntington's disease
• spinocerebellar ataxia type 1
• spinocerebellar ataxia type 2
• spinocerebellar ataxia type 3 (Machado-Joseph disease)
• dentatorubral-pallidoluysian atrophy
• Friedreich's ataxia

Typically the disorders are characterized by:

• amplification of the CAG repeat
• the CAG's are translated into tracts of glutamines
• small numbers of repeats in affected individuals, usually <100
• adult onset
• genetic anticipation is not as marked as in the mental retardation group; nevertheless, larger numbers of repeats result in early onset and severe disease

Friedreich's ataxia is exceptional:

• a GAA trinucleotide is expanded
• there is no clear genetic anticipation
• inheritance is autosomal recessive
• onset is earlier, typically in adolescence