Heterozygous, or "het" for short, indicates an unmatched pair of genes. Genes come in pairs for the most part (except sex genes), one copy from each parent. If a baby snake inherits a mutant copy of a gene from one parent and a normal copy of the same gene from the other parent it is said to be het for that mutation.
A snake is either het or it isn't, the "% het" nomenclature is to indicate the probability that it's a het when we can't tell for sure based on it's parents or it's appearance.
When you breed a het snake to a normal snake then each baby has a 50% chance of being het for the mutant in question. There is no guarantee that half the clutch will be het and half will not, it's a separate flip of the coin for each egg. It's not likely but they could all be hets or all not hets.
When you breed two het for the same mutation snakes together then each baby has a 25% chance of getting the mutation from each parent and being homozygous (matched pair of genes) mutant for that gene. If the mutation is recessive, then the homozygous babies look different and can be easily picked out leaving normal looking babies that are said to each have a 66% chance of being het.
The following rules for inheritance of genotype apply regardless of the mutation type (dominant, co-dominant, or recessive - but not sex linked which we haven't seen yet) and are the odds for each egg, not the guaranteed distribution of any clutch:
Homozygous X Homozygous = 100% chance Homozygous
Homozygous X Heterozygous = 50% chance Homozygous, 50% chance Heterozygous X Heterozygous = 25% chance Homozygous, 50% chance Heterozygous, 25% chance completely normal (once the homozygous are picked out that leaves 66% chance Heterozygous and 33% chance normal)
Heterozygous X normal = 50% chance Heterozygous, 50% chance normal
The mutation types of dominant, co-dominant, and recessive just tell you what the homozygous, heterozygous, and normal babies look like relative to each other.
With a fully recessive mutation the heterozygous babies are supposed to look completely normal (normal phenotype) and be indistinguishable from the normals. Only the homozygous animals look different. Albino is an example of a recessive mutation.
With co-dominant, the heterozygous animals aren't normal but the homozygous ones look different yet. Pastel is supposed to be a co-dominant mutation with the hets being pastels and the homozygous being super pastels. After years of only working with recessive mutations it's a common misconception that het means normal looking but it only describes the genes and with non-recessive mutations the hets are not normal looking. A pastel could be referred to as a het super pastel.
With a dominant mutation, just one copy of the mutation would be enough to make the animal fully mutant and the homozygous and heterozygous animals would look the same. Spider may turn out to be dominant and if so the only way to tell a heterozygous spider from a homozygous spider would be to breed a bunch of non homozygous spiders and produce only a bunch of spiders.
