In the future, some of the most serious health and welfare problems in horses may be caused by continued selection for certain appearance and performance traits Genetic traits are linked in complex ways and over-selecting for one trait causes corresponding changes in other traits. Likewise, over-selection for performance traits in racehorses and cutting horses causes changes to occur in other traits, such as temperament. Single Trait Selection Breeders of dogs, pigs, and cattle know that over-selecting for a single trait can ruin the animal. There are countless examples in the medical literature on dogs with physical and behavioral problems caused by over selection for a single trait. For example, breathing problems in short-nosed breeds, such as Bulldogs or Pugs, are caused by over selection for a shortened nose.
Pointer dogs with extreme nervousness are another example.
Pointing ability and nervousness are genetically linked traits. Breeders have known for decades that a fine line exists between a good pointer and a nervous wreck. Horses also can suffer negative effects with over-selection for both physical and behavioral traits. White piebald patches in Paint Horses, conformation and jumping ability in Warmbloods, conformation and speed in Thoroughbred race horses, and cow-cutting ability in Quarter Horses are examples of single traits that, when selected to the extreme, can cause problems in other traits. Single-minded selection for rapid weight gain in cattle, pigs, and chickens has already caused serious health and welfare problems.