As detailed above, silver (the inhibitor gene) is dominant while the recessive (hidden) version is golden. If a cat is not silver, genetically it must be golden. The phenomenon of a non-silver cat producing silver offspring when bred to an apparently non-silver cat has been documented several times. For example, Neils C Pedersen, Feline Husbandry, 1991 (pg 67) reported "several cases on record of black cats breeding as smokes." Robinson’s Genetics for cat Breeders and Veterinarians, 4th Edition, 1999 (pg 142) reported "occasional cats with no visible white undercoats that nonetheless breed as smokes.” Gloria Stephens’ Legacy of the Cat, 1989 & 1999 admitted "we do not understand about silver or the gene(s) that cause a solid-colored cat to be smoked.” The variability of smoke cats, ranging from poor smokes and hidden smokes to light smokes, is mentioned as far back as Frances Simpson in the early 1900s (this book pre-dates modern inheritance genetics so you have to rely on descriptions, some of her light smokes are evidently smokes). Some apparently solid cats are genetically smoke, but other genes prevent the pale undercoat showing up. However, could there be another form of silver, one that is either hypostatic (masked fully or partially by other colour genes) or is a second recessive allele of the inhibitor gene I and is recessive to both I (silver) and i (golden), but which produces a form of silver, perhaps i2 where dominance is I > i > i2.
All non-silver cats are, by default, by golden, though the golden will not show up in solid (non-agouti) cats. However, unexpected silver cats has turned up from time to time when two non-silver cats (by default golden) have produced silver offspring or a non-silver (golden) mated to a heterozygous silver (silver carrying golden) have consistently produced silver kittens, but non goldens even though the law of averages would expect half of the offspring to be non-silver.
What are the possibilities?
Mutation of the gene into the dominant inhibitor (silver) is plausible if it happened in the germ cells (ova, sperm) of just one cat. Germ-line mutations sometimes happen.
The supposedly golden cat (which should be homozygous for the golden form) might be a very tarnished silver due to other genes it inherited alongside the dominant inhibitor (silver) gene.
There might be a second recessive form at the I locus, one that is recessive to both I (silver) and i (golden), but which produces a form of silver. In a breed where silver is not permitted, the occurrence of unexpected silvers might be due to a 3rd allele that is recessive to golden.
A hypostatic gene for silver that manifests only when epistatic (masking) genes are eliminated. An epistatic gene is “dominant” over genes on other chromosomes (an example is “dominant white” masking other colours). Likewise a hypostatic gene is “recessive” to genes on other chromosomes. (I use the terms dominant and recessive outside of their strict meaning here)
2 separately inherited gene pairs (different loci) interact to produce the visual effect of silver. When inherited separately there may be no visual effect or a different effect e.g. lightening of the coat colour (“powder coat”).
In North America there is a phenomenon in some breeds called "powder coat" and "high colour." A powder coat means a lighter colour cat while high colour indicates a darker colour. This reflects variations in depth of colour. It is seen in solid colours such as cream, lilac and blue, but descriptions of variations in tone have been mentioned by Soderbergh (1950s) and Frances Simpson (early 1900s), especially in blues and the variations are generally attributed to polygenes. The inheritance of powder coats is predictable. In some Burmilla and Burmese lines, powder coats have been linked to unexpected silver offspring (mismating was ruled out) leading some to wonder if the powder coat and a hypostatic silver are linked.
Whatever the actual genetics turn out to be - and whether there is a single Wide Band gene of a handful of polygenes - the Silver and Golden series are attractive cats in both longhair and shorthair varieties.