Genes come in pairs with each parent contributing one copy to their offspring. With autosomal dominant genetic diseases only one defective gene is necessary to express the genetic disease in question. Such a gene can be inherited from either of the parents or from both parents. With recessive genetic diseases two copies of an abnormal gene (two copies = a pair) must be present for the horse to show signs of the genetic defect. WFFS is an autosomal recessive trait, meaning a foal can only be affected if the foal inherits the disease from both parents. Horses with only one copy of the defective gene are considered “carriers”.