N/N means he is negative for the gene
N/H means he is positive on one side and would be a 50% chance passed if the mare was also N/H. But a 25% chance if mare is not a carrier.
H/H is positive on both side and there is a 50/50 chance of passing the gene. A 75% if the mare is N/H and 100% chance if the mare is H/H
Forgive me, but I don't think that's right. Yes, an n/n horse does not have the HYPP gene, but those percentages seem off.
An HYPP n/n horse has no coppies of the HYPP gene. It is simply not there and they will never pass it on to their offspring. The N stands for negative, I believe. If you tested an Arabian for HYPP, they would be n/n. That does not imply that the horse has any connection at all with the disease.
If a horse is n/H, they have one copy of HYPP. If they are H/H they have two copies. An n/H horse always has a 50% chance of passing on one copy of HYPP and producing a positive foal. An H/H horse has a 100% chance of passing on one copy of the gene and producing a positive foal.
So if you bred an n/n to an n/H, 50% of the foals will be n/H. If you bred an n/n with an H/H, 100% of the foals will be n/H. If you bred an n/H to an H/H, 50% of the foals would be n/H and 50% of the foals would be H/H. If you bred an H/H to an H/H, 100% of the foals would be H/H.