Any decent biology textbook will tell you what you need to know about dominance, codominance and simple mendelian inheritance. Once you know that, simply need to know whether a particular trait is dominant, recessive, codominant, or something else. That information can generally be found on web sites of people who sell these animals.
Be aware however, that many people assume that just because a snake is different than its littermates, that it must be some sort of new recessive trait. This is, of course, totally unfounded, but a very common practice in the hobby among those with little background in Mendelian Genetics.
The 100% vs. 66% vs. 50% hets is simply a sales gimmick. It does not imply that a snake is 2/3 heterozygous (which is impossible) but rather that 2/3 of dominant phenotype animals should be heterozygous. Of course, there is no way to know which individuals are or are not, so if you buy these animals you are taking a chance.
To get these numbers you cross
Aa x Aa - gives you 66% possible hets because 2/3 of the dominant phenotype offspring "should be" heterozygous. Of course, they could all be homozygous as well. Simple probability.
AA x aa - gives 100% hets
Aa x aa - gives 100% hets (all dominant phenotype offspring are heterozygous - you will get some recessive phenotype individuals as well)
AA x Aa - gives 50% hets. All offspring will be dominant phenotype but have a 50% chance of being a het.
Do the Punnett Squares for these crosses and you will see where the probabilities come from. The probability is the answer to the question "If I choose one of the dominant phenotype individuals, what is the chance that it will be heterozygous?".
-----
Chris Harrison
