Huntington's only requires one parent to have a single dominant gene.
so if parent one is Hh and the other is hh, then there is a 25% chance the child will develop HD, which doesn't manifest until their 30's.
The disease is passed on because it doesn't manifest until the third decade and by this time most couple have already reproduced passing on their genes.
Now again, if both parents have a dominant gene Hh and Hh there's a 75% chance the child will be effected.
