Oh wow, LadyDreamer. I've never even heard of lordosis, and your post shows exactly what this thread should be about! Learning about new faults, conformational or otherwise.
I am now off to do some research on the matter!
In American Saddlebreds, they have just recently(last few years) found the specific gene marker for lordosis in American Saddlebreds. Funny thing is, horses can have the genes and not show it(THAT kind of low back is apparent as a baby) and not ever show it. I have seen mildly to moderately lordotic mares produce strong and straight backed colts.
What we believe(and something the researchers are looking into) is that environmental factors, such as developmental nutrition and the unnecessary stress of a major trauma play a huge role in it. The orphan suffered a traumatic birth, and did not receive everything he needed from his mother. He was given the best supplemental milk my parents could get 20+ years ago, but there was just something in his situation that caused his back to drop. His full brother, also a successful show horse was as straight backed as you could want. A very well made horse.
With the chestnut, again, no major flaws present in either parent. No foals on either side would have you believe the gene was there. His dam died before I knew her, but his sire was yet another we kept until he died of old age and was my best teacher. This horse when he was very young, days old I believe(I will have to ask my parents as the horse and I were born the same year. Lol.) took a thorn in the eye. This injury shocked his little system and shut his immune system down. He was very sick, and wasn't able to process the nutrients he was given as effectively as a healthy colt. Thanks to an experimental drug our vet was developing(which was successful) he eventually healed, but the stress of the situation, deficiency, and the right few genes caused it.
There was one mare I wish I had had the money to experiment with. She was a mild case who was a VERY hard keeper. She came to us in foal and lean and her filly was born low backed. I really wish I had had the money to get and keep her fat and in top health and see if she would have a straight backed colt. She died a few years after she was sold, so I never could.
They were perfectly sound to ride. Not too much out of kindness, and they were Jr. Exhibitor horses their whole careers. 90% of their show training/conditioning was done in the jog cart. They had special saddles and special padding, and they were good. I remember being put on the bay horse bareback when I was really little. It was like riding a reverse camel. Lol!
They are stellar equitation horses. They are a neat sensation to ride. They way their backs put you, you will not move. Your leg will be good, your posture good, and they will hold you in place. You would have to try to fall off. They all seem to have incredibly good personalities and temperaments too. I don't know if it has anything to do with the back itself or something else, but they are all just generally perfect in mind.
I find it really neat that horses can have such a severe spinal deformity and not have it effect their usability too much. A human with a mild spinal deformity can be super messed up both physically and mentally. Horses are so cool.
I hope that helped!