Spinal muscular atrophy
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Spinal muscular atrophy types I, II, III, IV and distal spinal muscular atrophy type 1 are inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.
X-linked infantile spinal muscular atrophy is inherited in an X-linked pattern. The gene associated with this condition is located on the X chromosome, which is one of the two sex chromosomes. In males (who have only one X chromosome), one altered copy of the gene in each cell is sufficient to cause the condition. In females (who have two X chromosomes), a mutation would have to occur in both copies of the gene to cause the disorder. Because it is unlikely that females will have two altered copies of this gene, males are affected by X-linked disorders much more frequently than females. A striking characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.
- Spinal muscular atrophy. Genetics Home Reference (GHR). December 2009; http://ghr.nlm.nih.gov/condition/spinal-muscular-atrophy. Accessed 5/13/2011.