Professional Herpetoculture for the Pet Trade

Genetics 401

While the previous pages have covered what most reptile keepers consider to be everything there is to know about genetics, in reality we have only covered the various modes of inheritance. Such a simplified understanding of genetics can lead to some serious errors. As our understanding of reptile mutations continues to evolve, we begin to see various outcomes of breeding trials which yield unexpected results. In reality, these results are perfectly understandable with an increased knowledge of the complexity of genetics.

Two important new terms must be fully understood to proceed:

Locus - Gene pairs in all life forms are always connected in a particular order. Each place in this order is referred to as a locus (as in location). It is very important to understand that while each locus accepts only two alleles, there may be several types of alleles which may fit at that locus. Thus many combinations of alleles could possibly be present at any one locus.

Allele - The proper term for what most breeders refer to as a 'gene'. The most common allele at any given locus is, of course, the 'normal' or 'wild-type'. An abnormal allele at any locus is usually referred to as a mutation. Remember that in most cases, both alleles at that locus must be mutated for the appearance of the animal to be altered. The exception would be dominant or incomplete dominant alleles.

Figure 1: 'Double Helix'

The famous 'double helix' (Fig. 1) represents the paired DNA strands, with thousands of connected pairs of alleles. While greatly simplified, try to imagine that one strand is provided by each parent and each bar represents a pair of joined alleles.

Think of it like this: All humans look pretty much alike because each locus stays in order, yet each human has recognizable variations as a result of the different alleles present at each locus. For example, there are several alleles available for the eye color locus. Thus we have friends with blue eyes, green eyes, brown eyes and so on - but they all have eyes because regardless of changes in the allele, the locus is still there. If we were to reduce the number of possible alleles at every locus to only one, all of us would be identical! (this is exactly what happens when a clone is created, every allele at every locus is identical to the original - not very realistic, but makes for some great sci-fi movies)

How does all this change anything?

Well, let me give an example:

Recently a number of Cornsnake breeders attempted to sort out the confusion of what appeared to be several types of hypomelanism and a strange looking new albino form derived from one of the types of hypomelanism. What was discovered could not be explained by conventional knowledge of genetics as understood by most reptile breeders.

It seems that one type of hypomelanism (trade-named Ultra) when bred to another of the same would produce more Ultra specimens. No surprise there. The surprise comes in when one of these snakes is bred to an amelanistic (Albino) specimen. Conventional wisdom would lead the breeder to expect all of the offspring from this crossing to appear normal, and each to be heterozygous for both Ultra and Albino. Instead, all of the babies appeared to be a strange new type of Albino (trade-named Ultramel). And when an Ultramel was bred back to either an Albino or or an Ultra, half the the resulting offspring looked like each parent! Using conventional 'reptile breeder knowledge' of genetics, this should be impossible!

What was happening?

It seems that the new form of hypomelanism (Ultra) shared the same locus as amelanism. They were different alleles residing at the same locus. Thus Ultra specimens have two copies of the Ultra allele at that locus, while the albino specimens have two copies of the amelanism allele at that same locus. Ultramel specimens have one copy of each type at that same locus. Many readers would ask "If these Ultramels have only one copy of each trait, they must be heterozygous for these traits, and why don't they appear as wild-type Cornsnakes?" The answer is simple: Since there is one copy of each allele present at the locus, there can be no copy of the wild-type allele at that locus to control the appearance!

While it's great to have figured this out, it's also forced another problem upon us. Reptile breeders have grown accustomed to using a very simplified method of representing the genes involved in these mutations with a single letter. This system of notation for genes (as used in our example Punnett Squares) does not take into account the concept of different alleles being able to reside at the same loci. It assumes that each loci can have one of two alleles, the normal or wild-type and the mutation involved with that loci. But as we've just seen, more than one type of mutated allele can reside at the same loci.

So a more proper form of notation is being offered to the reptile community. Initially set forth in the 2005 Cornsnake Morph Guide, this is NOT something recently made up. It's the way it SHOULD have been presented to the reptile community from day one. Many mysteries could have been avoided if this system had been in place.

It works like this: Large letters represent each loci, with smaller superscripted letters representing the alleles for that loci. Normal or wild-type alleles are represented with a +. An example for an amelanistic specimen: aaaa. using the previously accepted method of notation, it would simply be: aa.

A specimen heterozygous for amelanism would be represented as: A+aa.

While this may appear only to be twice the letters and nothing more, the real value can be seen when used to describe the various Ultra, Ultramel, and Albinos from the example above: aaaa= Albino; auau= Ultra; aaau=Ultramel. Notice that we can now see that all involve the same loci (a) but the different alleles can now be seen at that loci (a or u). Better still, we can now see the various allele combinations present and easily predict what offspring we will produce from any given pairing! This would be impossible using the older system of notation.

Beginning in 2005, VMS will begin using this system of notation for all Cornsnakes throughout our website. We'll implement the system for other types of snakes and geckos as well, once standards for such notation are agreed upon. This will enable novice and expert breeders alike to accurately predict the outcomes of breedings using specimens purchased from VMS.