In X-linked ichthyosis, a genetic defect, skin cells separate incorrectly at the outermost layer of the skin. The cells don't shed at a normal rate, and scales form. The defect occurs in approximately one in 6,000 births, but only in males, states the Foundation for Ichthyosis & Related Skin Types.
X-linked ichthyosis results in a deficiency of the enzyme steroid sulfatase, says FIRST. It is carried on the X-chromosome, and women may be passive carriers of the gene. The scales in X-linked ichthyosis are usually dark, and they are most commonly found on the back of the neck. X-linked ichthyosis is usually treated topically.