Horses in Legacy can inherit 6 different genetic diseases, 2 which have genetic testing available, and 4 which don't. Each disease has its own effects on a horse.
HERDA is an autosomal recessive disease caused by a mutation in the PPIB gene. It causes an excessive elasticity and fragility in the skin, and results in very poor wound healing. No treatments exist, and euthanasia is usually recommended.
In the game, a horse that is homozygous for HERDA (he/he) will die soon after birth. Heterozygous horses (N/he) are not affected.
SCID is an autosomal recessive disease caused by a mutation in the DNA-PKcs gene. Their B and T lymphocytes fail to mature, causing immunodeficiency.
In the game, horses homozygous for SCID (s/s) will be more prone to respiratory illness. This is separate from the other genes which can also cause a horse to be prone to respiratory illness.
These diseases are all rare, and are caused by recessive alleles. Because of this, you are likely only to see them pop up with inbreeding. But, because there is no test for them, they are harder to breed out unless you can trace their pedigree to find the original carrier!
Horses with hypohidrosis/anhidrosis don't produce adequate sweat to cool their bodies when exercising. As a result, they won't be able to train or exercise as long as other horses.
Horses with this disease will easily become obese. They will gain weight even on Basic feed! This can make them difficult to keep at a healthy weight, and they will be more likely to become obese and later develop metabolic laminitis.
Horses with asthma will breath abnormally loud when training. There is a treatment though! But, you will have to get it from your vet every month so it will cost you.
Keratomas are tumors in the hoof wall that can cause lameness. These develop in horses 5-8 years old, and will require surgery (a pricey one!) to remove. They will also need special, expensive shoes afterwards.