2 or 50% risk of inheriting the gene for HD. Each of his children is at a 1 in 4 or 25% risk of inheriting the gene, via your son, from his father. The fact that your son's child is gene negative could be because your son is also gene negative, in which case it is not possible for him to pass on the HD gene. However, he might still have the gene for HD, but  fortunately did not pass it on to his child. That is, the risk for your son of carrying the HD gene still remains 1 in 2 or 50% irrespective of the child's gene status. Had the baby been gene positive then your son must also have the gene for HD even though he isn't symptomatic as yet. If your son has a lesser risk, for example a grandparent had HD, but the at risk parent of your son  (i.e. either you or his father) doesn't have symptoms of HD and  hasn't been tested then your son's risk is less, in this case 1 in 4 or 25%. However, the fact that his child is gene negative still doesn't alter his risk. People also some times assume that if a person doesn't have symptoms of HD then they can't pass on the gene to their children. Unfortunately this isn't correct. The HD gene rarely becomes manifest until late 30s or older & so most people with HD develop symptoms after they have had children. The