If both the sire and dam are clear of the gene mutation (N/N), then there is no chance of an offspring inheriting the mutation. If a carrier of the recessive allele (N/FFS) is bred to a normal horse (N/N), each foal has a 50% chance of having two copies of the normal allele (N/N) and a 50% chance of being a carrier (N/FFS). If two carriers are bred, each foal has a 25% chance of having two copies of the normal allele (N/N), a 50% chance of being a carrier (N/FFS), and a 25% chance of being affected (FFS/FFS).
So within the warmblood population, assuming a conservative 8% carrier status, the following statistics could be assumed. Out of 100 broodmares, approximately 8 would be carriers of the WFFS allele. If those 100 mares were bred to a carrier stallion, approximately 50 foals would inherit carrier status (N/FFS), 48 foals would NOT inherit the recessive allele and would be clear of the gene mutation (N/N), and 2 foals would be positive for the disease (FFS/FFS).