This IS wrong. This only holds true if one of your parents is HETEROZYGOUS dominant. 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?