Enzymes are part of metabolic pathways and when they are unable to properly catalyze the chemical reactions they are responsible for, the person may have a debilitating condition or sometimes, it may be fatal. Tay Sachs disease is one example. In this case, two recessive genes are required for the child to have the disease. The baby is born and within five years or less, the child will die. This is because their body lacks heoxsaminidase A, a protein which is an enzyme necessary for the breakdown of a chemical in nerve tissue called gangliosides. Ganglioside, especially the one known as GM2 builds up in the nerve cells of the brain. Symptoms include deafness, blindness, loss of muscle strength, loss of motor skills, paralysis and seizures and death. Phenylketonuria or PKU is another recessive condition that occurs where a child cannot break down the amino acid phenylalanine. This is due to the fact that they are missing the enzyme phenylalanine hydroxlase, necessary for the breakdown of phenylalanine, found in protein foods. These substances build up in the body and lead to brain damage. The person must avoid most protein foods and eat a restricted diet, to avoid the brain damage that will result otherwise. These diseases are recessive and require two genes for a disease phenotype to occur. A person with only one recessive gene is merely an unaffected carrier.