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Homozygous DYT1 mutation carriers have not been reported in humans.

Does this mean that the mode of inheritance for dystonia (DYT1) is autosomal dominant? This to me implies that if one has both alleles for dystonia, then one must also have the phenotype, as people with both alleles cannot be carriers.

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Presumably homozygotes are not viable. –  Alan Boyd Jun 15 at 13:17

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Your description is correct. Most genes do not show complete dominance as outlined in Mendelian genetics and inheritance is almost always more complicated than a simple 1 gene/2 allele system. The mutation you're studying is classified as autosomal dominant but even heterozygotes may not display the phenotype. The proportion of the population that has the mutation AND displays the mutant phenotype is called the penetrance and, for DYT1, has been estimated at around 30%.

http://www.omim.org/entry/128100?search=Dyt1&highlight=dyt1

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