Reptile Genetics Part 1.

.... Introduction to the Punnet Square and the terms Heterozygous, Genotype, Homozygous, Amelanistic, Anerythristic, Double Recessive, Double Heterozygous

From this example we can predict that all hatchlings will be normal. Remember Capitals = Wild Type ( normal ) ( NN )

Normal X Normal = 100% Normal

Lets now look at crossing two Amelanistic animals, remember mutant ( recessive ) genes are expressed in lowercase and an Amelanistic was written as ( aa ).

From this example we can predict that all hatchlings will be Amelanistic. Remember lowercase = mutant, in our example Amelanistic ( aa )

Amelanistic X Amelanistic = 100% Amelanistic

Before we go any further we will explain the term Heterozygous or more commonly seen on price lists as het or hetero.

Heterozygous :

When we have one capital and one lowercase letters in the Genotype ( Na ) it's called herterozygous ("hetero" means different). ( Na ) is an example of heterozygous, the animal looks normal but carries the gene for Amelanistic. May be shortened to het or hetero on some price lists.

Amelanistic :

The Amelanistic gene prohibits the production of the black & brown ( melanin ) pigment. Also referred to as Albino, or shortened to Amel.

A heterozygous animal is one that carries a gene for a mutation such as Amelanistic. These snakes look normal, but carry a gene for Amelanistic. Mating a normal snake to an Amelanistic snake would give us heterozygous for Amelanistic. This is because the normal gene is dominant over the mutant gene.
We will now predict the outcome of crossing a Normal female to an Amelanistic Male

We know that the normal ( N ) gene is dominant so all our babies will look normal, but the lowercase ( a ) tells us that it is also carrying the Amelanistic gene. We would say these hatchlings are het for Amelanistic.

Normal X Amelanistic = 100% het for Amelanistic

Now lets take two of the above het for Amelanistic hatchlings grow them up and breed them together. So we have a female who is het for Amelanistic ( Na ) and a male het for Amelanistic ( Na ). Now because both parents can pass on either the ( N ) or the ( a ) gene we expand our Punnet square to include this.

We can see that breeding two het for Amelanistic together produces three different gene combinations.


25% NN = Normal
50% Na = Het for Amelanistic
25% aa = Amelanistic.

We are now faced with a problem, we can visually see that the Amelanistic's are different, but the rest of the clutch all look normal. We know through our Punnet Square prediction that 50% are het, but have no way of knowing which as they look identical to the Normals. In the introduction at the top of the page we mentioned 66% hets, well that is what the remainder of the clutch are described as. so if you see 66% hets on a price list now you know they were the hatchlings from crossing two het parents.Why 66%, take the Amelanistic away from the clutch, call the remainder 100% then two thirds or 66% actually 66.6% are het, so we say there is a 66% chance that this animal will be het.
A 50% het is the result of crossing a normal to het.

Our mutant example has been an Amelanistic, but it could of just as easily of been any single recessive gene such as Anerythristic or Hypomelanistic. If we were to predict any of the above outcomes using an Anerythristic animal we would use a different letter of the alphabet to that used by the Amelanistic. We could write the Anerythristic gene as ( rr ) and the Hypomelanistic gene as ( hh ).

Double Heterozygous :

When we have two capital and two different lowercase letters in the Genotype (RrAa ) it's called double heterozygous ("hetero" means different ). Our snakes will look normal but carry two different genes. May be seen on price lists abbreviated to dbl het.

Anerythristic :

The Anerythristic gene prohibits the production of the red ( erythrin ) pigments. May be abbreviated on price lists to Aner.

Our next example will look at the predicted outcome of crossing an Amelanistic female to an Anerythristic male. because both snakes have different recessive genes we use four letters for the Punnet square these are represented in both Capitals & lowercase, so our Anerythristic Male would be ( rrAA) and our female Amelanistic would be ( RRaa ).
Why do we use Capitals and lowercase well the ( RR ) in the Amelanistic calculation tells us that it has no Anerythristic gene, where as we know the ( aa ) tells us it's Amelanistic. The same is true but reversed for the Anerythristic snake the ( AA ) tells us there is no Amelanistic gene and the ( rr ) tells us it's Anerythristic. We can see there is only one combination of genes that can be passed over from each animal in the Amelanistic it is ( Ra ) in the Anerythristic it is ( Ar ).

In this example we can see that breeding an Amelanistic female to an Anerythristic male produces just one type of hatchling. Because both lowercase letters are different we know it is not an Amelanistic or an anerythristic, for it to be a mutation both lowercase letters would

have to be the same eg: ( aa ) or ( rr ). So our hatchlings must be wild type or normal. The lowercase letters tell us it is a het, but this breeding has produced two types of het, Amelanistic ( a ) & anerythristic ( r ). Our babies are called Double Het for Amelanistic & Anerythristic, You may also see this type of snake advertised as het for Snow or Double het for snow. Why snow? if because when breeding two of our double hets together there ia a chance of producing a snow or a double recessive as it is sometimes called.

Double Recessive :

When we have no capitals and four lowercase 2X2 letters in the Genotype ( rraa ) it's called double recessive. Both sets of genes will have an influence on the appearance.

When breeding two double hets together to hopefully produce our snow ( double recessive snake ). There are four possible combinations of genes that can be passed on in this breeding. ( RA )  (Ra ) ( rA ) ( ra ). So lets put them in the Punnet Square and predict what we could breed.

We can see that 9 of the 16 will appear normal, but some of these ( 8 ) will be either het for Amelanistic ( 2 ) or Anerythristic ( 2 ) or both ( 4 ) ( double hets )


Three of the hatchlings will appear Amelanistic ( RRaa ) but 2 may carry the Anerythristic gene ( Rraa ).


Three of our eggs will contain Anerythristic babies ( rrAA ) which 2 may also be carrying the Amelanistic gene ( rrAa )


One of the 16 will be a snow ( rraa ) a double recessive mutation. The snow is expressing both the Amelanistic & Anerythristic gene.


The Amelanistic gene prohibits the production of the black & brown ( melanin ) pigment.
The Anerythristic prohibits the production of the red ( erythrin ) pigments. So our snow lacks brown, black & red pigments, which leaves us with a very pale snake, in shades of white some yellow may still be present.
As you can see it is a long process to produce snows, then there is only a 1 in 16 chance of producing one, no wonder when they are first produced  they command high prices.