Some aspects of an animal are determined by their environment, their experiences, their diet, or even accidents (perhaps during development) but many are determined by their genes.
With few exceptions, genes are found in pairs, with one member of each pair inherited from mom and the other from dad. There are a great number of different genes doing a great number of different things. Every now and then a mutation happens that changes what one of those genes does in such a way as to make a different (“mutant”) looking animal. With the huge number of ball pythons exported from their native Africa each year (as many as 150,000 some years) we have had the opportunity to discover quite a variety of mutant genes. We then breed these odd looking animals in captivity to confirm if the odd appearance is genetically reproducible and if so how it works and how it might combine with other mutant genes.
A few basic terms are helpful to describe the possible inheritance of an unusual ball python appearance:
Phenotype: The appearance type of an animal. Ball python phenotypes include normal/wild type (which can vary a lot), albino, pastel, super pastel, spider, and many many more. The phenotype doesn’t always tell you about an animal’s genes and in some cases some recognized phenotypes might not even have a genetic cause. For example, the recognizable classic jungle appearance might be caused by incubation environment — or it might be complex genetics we just haven’t figured out yet.
Genotype: The gene type of an animal with respect to one or more known genes. Remember that genes come in pairs (except for gender genes of the mismatched sex – females in snakes). One copy of each gene comes from each parent. We really only know ball python genes with respect to the mutations we have identified so far. As far as we know, there are only two versions of the gene at the location we refer to as the “albino locus” – albino mutant and normal for the albino gene (without the albino mutation). Depending on which version of this gene a baby ball python gets from each of it’s parents it has three possible genotypes for the albino locus – homozygous normal (two normal for albino copies), heterozygous albino (one normal and one albino mutant copy of the gene), or homozygous mutant (an albino mutant copy of the gene from each parent).
The genotype can be expanded to also include other gene locations, for example an axanthic gene. A double het for snowball python is both heterozygous for the albino gene (one albino and one normal for albino gene at the albino locus) and heterozygous for axanthic (one axanthic and one normal for axanthic gene at the separate axanthic locus).
Mutation Type: The third important concept for understanding ball python mutations is classifying the interaction between genotype (the genes) and the phenotype (the appearance). There are three basic categories of mutation types.
With recessive mutations (albino for example) just one normal copy of the gene is enough to compensate for one mutant copy of the gene. In a heterozygous albino the one normal for albino gene inherited from one of the parents covers the one albino copy of the gene from the other parent and makes the “het” albino look normal.
In a codominant or incompletely dominant mutation (there is a technical difference and debate continues on which is correct for certain ball python morphs) the one mutant copy in a heterozygous animal produces a visible mutant phenotype but the homozygous mutant version is a different (usually more extreme) phenotype. A heterozygous for pastel genotype ball python has the pastel mutant phenotype but a homozygous for pastel genotype ball has the super pastel phenotype.
A third mutation type is completely dominant. You will often see this type referred to as just “dominant” but I prefer to add “completely” to avoid confusion as “dominant” is also often used for the super category of all mutations that have been proven non recessive but may yet prove codominant. With the proven completely dominant mutation type, even one mutant copy would completely cover the normal version of the same gene. Some believe that Spider will prove to be completely dominant and that the homozygous spider genotype will have the same spider phenotype as the heterozygous spider genotype. If this is the case you can think of it as one spider gene dominating one normal copy of that gene and being enough to make the snake look just as fully spider like as one that has two mutant copies of that gene.
So, the quick and dirty definitions of the three mutation types are:
Recessive: The heterozygous genotype looks normal and only the homozygous genotype is a mutant phenotype.
Incomplete Dominant: The heterozygous genotype is a visible mutant phenotype but the homozygous genotype is a different visible mutant phenotype.
Dominant: The heterozygous and homozygous mutant genotypes are the same mutant phenotype.
As you start to consider predicting the outcome of increasingly complex combinations of varying mutation types I find it best to follow these steps:
Chances are you already know these 5 breeding results from the recessive morphs you may well have learned about first. All you need to learn is to break the dominant mutant type phenotypes down to the correct genotype equivalents and you are ready to predict the outcome of crosses that will confound your friends who still think “het” only applies to recessive mutations!