List and describe the four X-linked recessive disorders.

1. Color-blindness results in color vision deficiency. The most common of the sex-linked forms of color blindness is red-green color-blindness.
2. Duchenne muscular dystrophy is characterized by wasting away of the muscles. Symptoms include a waddling gait, toe walking, frequent falls, and difficulty in rising; may appear as soon as the child starts to walk.
3. Fragile X syndrome is caused by an abnormal number of repeat sequences in the genome and is the most common cause of inherited mental impairment.
4. Hemophilia is called the bleeder's disease because the affected person's blood either does not clot or clots very slowly.