Corn snakes have two copies of (almost) every gene. For each gene, there can be many alternative forms which are called alleles. Many times a gene is named after the first mutation discovered for that gene. For example, the amel locus (locus and gene are often used synonymously, but a locus is technically the location in the chromosome where a particular gene resides) can be occupied by the normal allele, the mutant amel allele, or the mutant ultra allele. For the sake of argument suppose that the amel locus is on chromosome 6. In this hypothetical situation, a corn snake has two copies of chromosome 6 and each chromosome 6 has an amel locus. But the chromosomes don't have to have the same alleles. One chromosome 6 could have the mutant amel allele and the other chromosome 6 could have the normal or "wild type" allele at the amel locus. Or a corn could have one ultra allele and one wild type allele. A corn snake that has one ultra and one amel allele will have the ultramel phenotype, which is intermediate between wild type and amel.
Because amelanism is recessive, both alleles at the amel locus have to be the mutant amel form for the snake to appear amelanistic. A snake that has two wild type alleles at the amel locus will look normal, and so will a corn snake that has one amel allele and one wild type allele (these corns are referred to as "het amel"). When a corn snake reproduces, it can only pass one of its two alleles for each gene to its offspring. So if a corn snake has one amel allele and one wild type allele, it will randomly pass down one of those alleles to its offspring (so the odds of it passing down an amel allele is 50%, and also 50% to pass down the wild type allele).
This is why breeding an amel to a ghost will not give you a coral snow. An amel corn has two copies of the amel mutation but it has 2 wild type alleles for both hypo and anery unless it happens to be het for one or both of those genes. A ghost has two wild type alleles at the amel locus, but two mutant alleles at both the anery and hypo loci (plural of locus, pronounced "low sigh"). When you cross them, you will get snakes that are het hypo, het anery, and het amel. The offspring of a cross are termed the F1.
Think of what happens when you cross these F1 snakes together to produce F2, and what the odds are of getting 2 mutant amel alleles, 2 mutant anery alleles, and two mutant hypo alleles all in the same baby. Since each F1 snake has a 50% chance of passing on a mutant amel allele, a 50% chance of passing on a mutant anery allele, and 50% chance to pass on a mutant hypo allele, the odds of getting an amel anery hypo are (1/2)^6 or 1/64. This is because the F2 snake in question has to get the mutant allele for each of three different genes from BOTH of its parents, which involves 6 events that each have a 50% chance of occurring. So the odds are 1/2 (odds of getting amel allele from mom) x 1/2 (odds of getting amel allele from dad) x 1/2 (odds of getting anery allele from mom) x 1/2 (odds of getting anery allele from dad) etc for hypo.
Strawberry is one of the many alleles of the hypo gene. The hypo locus has at least 4 different alleles, but any individual corn snake can only have two at a time. Those alleles are wild type, hypo (the first mutation of this gene discovered), strawberry, and christmas.
All of the mutations in your corns are recessive. So you could breed your butter motley to your amel motley and get amel motleys het caramel. Breeding your ultramel anery to your butter motley would yield ultramels het anery caramel motley and amels het anery caramel motley.
I've done a total hack job explaining this via typing on my cell phone but hopefully it made at least a little sense!
