Huntington's disease is a neurodegenerative disorder and its symptoms are adult onset and begin to appear between the mid-thirties to mid-forties. It is an autosomal dominant gene that produces this disease. This means that if an individual inherits only one mutant allele from one parent and the other allele is normal, the person will develop Huntington's disease. The mutant allele produce an abnormal version of the protein Huntingtin which begins to damage areas of the brain. Tay Sachs disease is a genetic disease that occurs when the child is born. It is an autosomal recessive condition and the person must inherit two defective genes for a disease phenotype to occur. In Tay Sachs, gangliosides accumulate in the nerve cells of the brain, causing the child to deteriorate both mentally and physically. Death occurs before or up to age four. It is prevalent in Eastern European Jews. Phenylketonuria or P.K.U. is an autosomal recessive condition therefore an individual must inherit two genes for the disease to occur. An individual with this disease cannot metabolize phenylalanine to tyrosine, which is a metabolic pathway that must occur. Because of this, phenylalanine accumulates and is metabolized to phenylpyruvate. This can lead to mental retardation, seizures and brain damage. However, if a baby is tested and is known to have this disease, a restricted diet free from the amino acid phenylalanine as well as strict monitoring and other supplements, will slow or stop the progression of the disease.