Ball Pythons and Corn Snakes have quickly become the white mice of the snake world.  Both have so many genetic mutations, that you can find individuals in virtually any pattern and color combination. Genetics can be confusing to many individuals.  We will attempt to present the topic in an easy-to-understand way. We have included a glossary for your convenience.  As we introduce terms, you may click over to the glossary to help you understand the concepts.Like all living organisms, changes in the DNA make individuals unique.  If the changes are subtle, it makes us what we are, tall or short, green eyes or brown. Occasionally, there is a genetic mutation that produces a quite pronounced phenotype – for instance an albino snake. The reptile community has embraced these mutations, and brought them to the marketplace through selective breeding. 

In its simplest form, a mutation can be either dominant or recessive.  When for a single mutation, these are called alleles.  If the animal containing a dominant mutation is bred to a normal individual, half the babies will show the dominant phenotype, even though they received genetic information from the normal animal.  

Geneticists predict the gene combinations (genotypes) and the resulting physical characteristics (phenotypes) using a tool called a Punnett Square. It is a diagram designed by Reginald Punnett and used by biologists to determine the probability of an offspring having a particular genotype. It is made by comparing all the possible combinations of alleles from the mother with those from the father. In a mutation the dominant allele is characterized by a capital letter and the recessive a small letter.  As an example, let’s take the case of the Spider Ball Python, a dominant mutation.Since the mutation is dominant over the normal allele, it would be notes as “S” and the normal “s”. Since it is dominant, you cannot tell by looking at the animal whether the alleles are homozygous (SS) or heterozygous (Ss).  Therefore you will perform two calculations to predict what the offspring of a Spider x Normal would look like. 

Dominant Mutations 

First assume the Spider is heterozygous (Ss) for the trait. Take the two possible alleles that the Spider can contribute (S and s) and place them at the top of the square. Since the normal has no spider mutation it will contribute “s” only.  This goes on the left side of the square. 

Take the letters from the top and sides, and add them together in the boxes. Since Spider is dominant, Ss will give you Spiders, ss will produce normals.  Thus a heterozygous Spider to normal breeding will produce 50% Spiders and 50% normals.Now, assume that the Spider is Homozygous for the mutation (SS). There is only one possible allele (S) for the Spider, and one for the normal (s)  Using the Punnett square, you get this:  

Incomplete Dominant (Codominant) Mutations 

Next, is the situation of incomplete dominance. The most familiar examples are Pastels and Lesser Platinum ball pythons.  This is a dominant mutation, but the heterozygous (Ll) (lesser Platinum) form looks different than the homozygous dominant (LL) (Leucistic). Here is a Punnett Square  showing a cross of two Lesser Platinum (Ll) animals: 

Recessive Mutations 

The final situation is the recessive gene.  The most familiar here are the albino, piebald and ghost.  As the name implies, the normal gene is dominant over the recessive, so the only time you will see the albino is in the homozygous recessive state (aa). This means that you will never see an albino individual on the first generation cross (F1) with a normal individual.  Here’s how it looks on a Punnett Square: 

Carrying this to the second generation (F2), by crossing the Heterozygous animals. We get the following: 

This is the square that gives rise to the “66% Het” scenario. You will notice that this cross produces one normal and two heterozygous individuals.  However, all appear normal visually. Of the three normal looking animals, two are heterozygous, but you don’t know which ones they are. Thus, each of the normal looking animals has a 66% chance of being a heterozygous individual. 

Finally, keep in mind that these are purely statistical probabilities.  It is theoretically possible to cross a Lesser with a normal and come out with all normals.  It is also possible that this happens in one, two, or more clutches. 

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