How are the cause and onset of symtoms of Huntington's disease different from those of PKU and Tay-Sachs disease?

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.