As with all medical conditions, some congenital defects are more well-known than others. Congenital heart defects are a wide category, encompassing many structural anomalies.  Cleft lip and cleft palate occurs when the tissue of the upper lip or the roof of the mouth, respectively, fail to form.  Today, surgery can correct many heart defects, cleft lip, and cleft palate.  Spina bifida occurs when a cleft forms on the spinal cord, preventing it from closing. Depending on the location of the cleft, individuals may face partial paralysis and hydrocephalus, where fluid collects around the brain.  Club foot is a congenitally malformed foot.  Finally, Down syndrome appears as a group of symptoms, including mental retardation and facial characteristics.  It has been linked to a third 21st chromosome.