That doesn’t make you an expert in genetics. The OP is opining that there is some genetic thing unique to the Impressive line that makes his offspring unpredictable and over reactive.
In reality, horses that seem “unpredictable” just have unobservant people around them. They internalize stress and don’t react much to what is going on around them until they reach a certain point, and then that “quirk” or whatever switch gets flipped and you get some crazy behavior or what not. People who are more observant will see that even though the horse is controlling himself and not reacting to what is going on, he is still stressed and getting anxious. If you don’t do anything about it, then sure when you finally get a reaction, it “seems” overly reactive and stupid.
I had an Impressive horse as a boarder and I loved him. I think his full name was Chubby’s Impressive and he was owned by a man who was a real beginner, but loved his horse. I remember the horse as very curious, bold, smart, incredibly patient with his owner and such a character, he once climbed over a huge stone wall just to get to the other side and torment the horses in his huge 6 acre pasture with a ‘see what I can do?’ attitude. He was a funny, sweet horse.
Even when you CLONE a horse several times, you get two VERY different animals. Yes you can breed and cross your fingers and sometimes get what you want, and have a higher probability through breeding, but there is so much we don’t know about which genes are relevant, how they are inherited, how they are expressed in every life stage from fertilization to in utero to early interaction with mom, etc etc etc.
i guess my point int is anyone going into a conversation with someone else, “hey, my horse is Impressive 4 generations back and is trigger happy and tricky” is going to elicit like responses because humans look for patterns and familiarity to double down on preconceived notions. From the examples in this thread, no not all Impressive descendent are like that but you see people ignoring that to argue that their horses who ARE tricky sensitive whatever are like so Bc of their Impressive heritage.
I think it can be both. We had a racemare who as crazy talented, but also just crazy. Could. Not. Stand. Dirt in her face. and just too hot to race. She was so talented and well made and well bred that we bred her, with much success, but all of her foals were sensitive and needed to be carefully managed as to not follow in their mom’s footsteps. Three of them had the exact same “NO DIRT!!” quirk. I think our trainer, having trained the dam, handled her offspring a bit different but the propensity was 100% inherited.The nice thing is, they also inherited her beautiful gait, huge set of lungs and die-hard try.
On the plus side, they were all very sweet and easy around the barn (including their dam), and the ones we retired and adopted out to riding homes all transitioned beautifully. Their dam still ambles around at 20, fat as a hippo. We’re hoping to breed her granddaughter (the daughter of her world champion daughter) next spring - she didn’t catch this year.