Dead or deformed foals produced by purebred parents represent an economic and emotional loss to the horse breeder. In order to avoid producing such defective foals in the future, the breeder may seek guidance in determining whether their origin is environmental or genetic. Only a few genetic diseases of horses have been rigorously defined. Selected, rare genetic diseases that prevent reproduction, cause the natural death, or necessitate the humane destruction of a foal before it fulfills its intended purpose serve as examples for collecting evidence to define the genetics of other deleterious diseases in the horse. In advising breeders how to avoid genetic defects in future foals, the professional needs to verify with appropriate evidence that the defect is genetic in origin, to discuss the mode of inheritance with the owners, and to advise them of the findings in writing so that they have a reference for making decisions.