Hunter syndrome is inherited as an X-linked recessive disease. This means that boys will be the ones most often affected, because the defective gene is on the X chromosome. Boys have only 1 copy of the X chromosome. Girls have 2 copies. Because girls have two X chromosomes, their normal copy on one X can provide a functioning gene, even if their other X has the abnormal gene. Women can carry the defective gene and pass it on, without being affected themselves, unless both their copies are abnormal. But because boys have an X and a Y, there is no normal X gene to fix the problem if the X is defective. The metabolic abnormality that causes Hunter syndrome is a lack of the enzyme iduronate sulfatase. Without this enzyme, mucopolysaccharides collect in various body tissues, causing damage. Affected children may develop an early-onset type (severe form) shortly after age 2 that causes a large skull, coarse facial features, profound mental retardation, spasticity, aggressive behavior, joint stiffness, and death before age 20. A late-onset type (mild form) causes later and less severe symptoms. | 
Reproductive Genetics
