There is another very well documented phenomenon among the Roman snail. Among thousands of specifically dextral shells there may be one or the other rare sinistral specimen. Those then are colloquially referred to as snail kings.
The coiling direction of a snail shell's whorls is determined genetically. Because of the rarity of sinistral Helix shells one would certainly assume, that shell genetics is based on a normal dominant-recessive pattern of heredity. It may, though, happen, that after the mating of two dextral Helix snails the offspring is exclusively sinistral. That is impossible, if one assumes the pattern of heredity to be conventionally dominant-recessive.
Because of geneticist Alfred Henry Sturtevant discovery, we know today, that shell coiling in gastropods is indeed inherited in a dominant-recessive pattern. The direction, in which a snail's shell whorls finally coil, however, depends on the direction of cell cleavages in the fertilised egg cell. So the genetic information that determines a snail shell's coiling direction can not be found in the snail's own cells, but in those of the mother animal - in the case of the hermaphroditical land snails the snail the egg cell was from.
That means, the 'mother's' genome is important to a snail's coiling pattern, not its own. Snails that are externally dextral (that is called the phenotype) may have exclusively sinistral offspring, because they are genetically sinistral (that is called the genotype). And besides their genotype must, of course, be homozygote, because the character "sinistral" is the recessive allele.
Living snail king, though, are very rare (one in several thousand) and almost never encountered in nature.
The phenomenon of organs situated on the side of the body opposing the natural location, a part of sinistrality in snails, is also an illness known among humans. It is then known as situs inversus.
Snail King (on the right)
Snail King while Hibernation