This IS wrong. This only holds true if one of your parents is HETEROZYGOUS dominant.If one of your parents has Huntington's disease, you have a 50% chance of getting the gene for the disease.
Hh x hh => 50% chance of offspring having Huntington's disease.
However, if one of your parents is HOMOZYGOUS DOMINANT ..
HH x hh => 100% chance of offspring having Huntington's diseases.
Simple Punnett squares will verify the above.
Therefore, the probability should be 75% if one parent has Huntington's disease. Am I right?