Occurrence of familial spastic paraplegia in only one of monozygous twins.

Three patients who suffer from spastic paraplegia are described who belong to two generations in one family. One of the patients, who has had symptoms and signs for at least 10 years, has a monozygous twin who is unaffected. Using blood groups and chromosomal polymorphisms, the probability of monozygosity is estimated to be 0.99986. The observation of nonpenetrance in familial spastic paraplegia suggests that environmental factors may be involved in provocation and emphasises the need for careful genetic counselling in this and related diseases.