Huntington's disease is caused by the trinucleoide CAG repeats in a gene coding a protein called huntingtin. As often happens with trinucleotide repeat diseases, the syndroms get worse over generation. This effect is called genetic anticipation and Huntington's disease is not an exception. As the altered HTT gene is passed from one generation to the next, the size of the CAG trinucleotide repeat often increases in size. The longer the size of the repeat, the more earlier onset of signs and more severe symptoms.
Kaiser Faward, Genetic anticipation in Huntington's disease, 2004