From what I have read if you keep the stress level low and the feed correct, your filly may never exhibit the HYPP because she is heterozygous for the gene.
She is very halter bred.. but I remember some of those horses and I can say that they did not all look like the diaper butt awful straight pasterned tiny hooved things you see today.
IOW's if she takes after some of Impressive (in looks) with some Skipa Star in there she may be just fine.. and look and work well.
I remember Impressive.. and while he was a halter horse he did live up to his name.. genetic mutation and HYPP and all that.
If your filly is a little thinner and a little more athletic.. well three cheers for you!
There is something about the outside of a horse that is good for the inside of a man. ~Winston Churchill
) Dinosaur Horse Trainer