The Labrador has been added to the list of breeds that can be tested to determine whether they carry the gene for long-haired coat. Long-haired coat length is inherited as an autosomal recessive trait. Therefore, dogs that are carriers of the long-hair mutation will appear to be normal (short hair) themselves, but will likely pass on the long-hair mutation 50% of the time.
Here is some history that I was able to glean from a few web sites:
The Labs behind the "quirk/throwback" in Switzerland were imported some 30 years ago. The two imports became champions, one became the Bitch World Champion with two puppies (one definitely carrying the gene) from her first litter gaining the titles of Junior World Champion.
A possibility is that the gene comes from the St. Johns dog, a progenitor of the modern Labrador. The drawing of the St. Johns dog portrays a shaggy dog with a long coat and the actual "Breed Points for the St. Johns or Labrador Dog", published in 1879 called for a very different coat than the one we now see as typical. Here is the quote:
"COAT is moderately short but wavy, from its length being too great for absolute smoothness. It is glossy & close, admitting wet with difficulty to the skin, owing to its oiliness, but possessing no undercoat".
Actually, one of the factors that influenced the formation of the LRC of Great Britain in 1916 was the problem of inter-bred Retrievers. Up until 1917, the KC allowed registration under the breed they most closely resembled and the 1915 Lab Crufts winner had a sire that was a purebred Flatcoat with relatives in that breed's ring.
Other thoughts from the LRC, Inc. are that particular dogs and particular bitches carry the same gene perhaps linked to an interbred retriever as far back as the 1930's or farther:
"It was not uncommon in the early days of the breed and after WWI to use, for example, to use the offspring of a Flat-Coated Retriever x Labrador Retriever cross to refine or adjust a trait in the Labrador Retriever.
Many old pedigrees note that a particular dog or bitch was "interbred". One of the well known Labradors imported into the United States in the 1930's had an interbred bitch in his pedigree 3 generations behind him. Those interbreds were probably short coated but may have carried a gene for long coat. So when two Labradors who carry the common gene are bred, it is more than likely a "long hair" throwback will result.
You will not find references to "long haired or long coated Labrador Retriever" because the trait is not part of nor recognized in the breed and breeders would quickly eliminate a Labrador that would produce the trait from the gene pool."