A normal woman has two X chromosomes; a normal man has one X an Y chromosome. A normal appearing woman can carry a defective recessive gene on one X chromosome and canpass it on to her children. Each son of a carrier mother will have a fifty/fifty chance of manifesting the X-choromosome disorder, while each daughter will have a fifty/fifty chnace of becoming a carrier like her mother, unexpectedly by the disorder herself but capable of passing it on to her male children.

There are soX-linked disorders, however, in which female children can also be affected by the disease itself. There are about 240 disorders known to have opr suspected of having transmission by X chromosome genes. They include hemphollia, color blindness, and agammaglobulinemia, a lack of immunity to infection.