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This chapter reviews the major advances in autosomal recessive and autosomal dominant ataxias, discusses the use of genetic tests in these disorders, and summarizes some current ideas regarding pathogenesis. It also presents a list of the autosomal recessive ataxias that have been genotypically characterized to date. Mutations in ataxia with isolated vitamin E deficiency (AVED) are scattered throughout the gene and some of them may be associated with a mild phenotype, late onset, retinitis pigmentosa, and retained reflexes. A syndrome of ataxia associated with optic atrophy, visual loss, and cochlear degeneration has been mapped to chromosome. The spinocerebellar ataxia (SCAs) exhibits many phenotypic similarities so that it is almost impossible to diagnose the genotype from the phenotype alone. Many persons from families with ataxia will request predictive testing and occasionally prenatal testing. Disease-modifying therapies are under investigation and include antioxidants and drugs that may modify excitotoxicity or apoptosis.
The genetics of a disease such as amyotrophic lateral sclerosis (ALS) require some flexibility in thinking about familiality compared with the genetics of isolated cases. About 10% of the time, an individual who develops ALS also has first-degree relatives who have been affected. For the remainder, the disease is said to be sporadic, but detailed investigation of the family tree may occasionally reveal that cousins or other more distant relatives have been affected. ALS can therefore be seen as a disease with an inheritance pattern that lies on a continuum from sporadic disease, to familial clustering to clear Mendelian familiality. For simplicity we have maintained the conventional separation of familial and sporadic disease, but as will become obvious, this distinction is largely artificial.
Familial ALS and the first descriptions
The concepts of genetics were coming into being at about the same time as the earliest descriptions of motor neuron diseases. Mendel presented his classic paper in 1865 (Mendel, 1865) in which he described his famous sweet pea hybridization experiments. He did not use the term gene or genetic to describe the heritable units but called them “formative elements.” Cambridge Professor of Biology William Bateson coined the term “genetics” (from the Greek “to generate”) in 1905 when applying for a university chair in a letter to the Cambridge zoologist, Adam Sedgwick. He wrote, “Such a word is badly wanted and if it were desirable to coin one, Genetics might do.”
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