Miss knowitall, you probably had no clue yourself when you had your first litter with this particular breed.

Variations of the agouti allele (bicolor, saddle pattern, etc.) are created because of mutations that inactivate series of codons that would normally complete the expression of the original wild type sable coloring.  There are known incidents of reverse mutation, where these codons can regain their function.  The loss of function mutation that would cause black coloring has been narrowly pinpointed on the agouti allele, causing the entire allele to lose function.  Inactivation of other portions of codons of the allele explain the myriad of other expressions we observe.  A possible example of reverse mutation, causing gain of function (GOF) expression would be Dolf z Eurosportu.

Any workingline breeder should know, when you breed two doorknobs together, there's a reeeally slim chance of getting an exceptional performance animal out of the deal.

For a period of 3 years, Gunther Schlager and Margaret Dickie estimated the forward and reverse mutation rates for five loci in mice that encode various aspects of coat color by examining more than 5 million mice for spontaneous mutations (G. Schlager and M. M. Dickie. 1966. Science 151:205–206). The numbers of mutations detected at the dilute locus are as follows:

                            Number of gametes examined     Number of mutations detected

Forward mutations               260,675                                           5

Reverse mutations               583,360                                           2

"Mutations can be reversible. If a mutation occurs once in a gene, there is a very small probability that the mutated base could mutate back to its original form. Alternatively, there are occasions when a mutation in a second, separate gene will return the phenotype of the organism to a wild type appearance (a rare case of two wrongs making a right). This kind of mutation is known as a supressor mutation."