As with many of the hardest to eradicate genetic conditions Fell Pony Syndrome is linked to an autosomal recessive gene. This means that the condition will only occur if both the dam and the sire have the gene but are only carrying one, not two copies. When the two mate, each contributes one half of the double gene needed, and if both contribute the recessive gene for the syndrome the result is a foal that will only live a few short weeks before dying.
Autosomal means that there is one, two-part gene that is responsible for the condition. The foal will inherit one part of the gene from the mother and one part from the father to make the complete gene pair. Since foals with the condition will die and not reproduce, only carriers, adults that have one Fell Pony Syndrome gene and one normal gene, or clear, adults with two normal genes, will continue on in the population. Since horses with only one gene have no visible symptoms of carrying the gene, they can only be detected through careful monitoring and recording of breedings that produce either healthy or unhealthy foals.
Because of the laws of genetics, even two carriers that are bred together have only a one in four chance of producing a foal that will die of Fell Pony Syndrome, a one in four chance of producing a foal that has no recessive genes for the condition and a two in four chance of producing a foal that will be a carrier as well. This does not mean that one out of every four foals produced by the same dam and sire will die of Fell Pony Syndrome, it means that each foal has a one in four chance of carrying the double recessive.
Fell Pony Syndrome includes the inability of the foal to develop an immune system, which results in death from any one of the many viral infections of the respiratory system or metabolism. Since the foals are unable to generate antibodies and there are no "cures" or vaccinations for viruses other than to stimulate the body to make antibodies, these foals simply cannot survive. Commonly foals will die before the first month after birth, no matter what the owners do to assist the foal. The good news for Fell Pony owners is that if the foal survives past the first month with no signs of illness or respiratory problems he or she is either clear, having no genes for Fell Pony Syndrome or is, at the worse, a carrier that will have a normal life.
Since the Fell Ponies have been isolated for most of their development, the recessive gene is fairly prevalent in the population and is estimated that approximately sixty to ninety percent of the adult Fell Ponies carry the gene, while approximately twenty percent of all Fell Pony foals will die of the condition in any breeding season. Ongoing research in the United Kingdom is key to identifying the genetic tracer to identify clear or carrier horses and try to work towards a comprehensive breeding program. Since a clear Fell Pony can breed with a carrier and always produce a unaffected foal there is hope to continue to expand this rare breed while avoiding contributing to the number of foals born with Fell Pony Syndrome.