How Het Percentages Work

It is very common to see a reptile for sale with a label that reads "50% pos het Albino" or "66% pos het Clown". These percentages cause a good bit of confusion for people who are just beginning to think about the genetics of their animals and new breeders who haven't brushed up on genetics 101. "Pos Het" is short for "Possibly Heterozygous", which is a way of saying "this animal may or may not have the following gene". Genes come in pairs, called alleles, and each pair is located in a specific position (or locus) on a chromosome. If the two alleles at a locus are identical to each other, they are homozygous; if they are different from one another, they are heterozygous. You will only see these 'hets' listed when a recessive gene is at play. A recessive gene needs to be homozygous 

So why 50%, 66%, and 100%? These percentages come from the Punnett square, a tool used to calculate the probability of offspring inheriting different traits from the parents. In the punnett square, each parent is represented by a pair of letters which in turn represent either having, or not having the trait being calculated. I will use albinism as an example for the rest of this post. In this case, a capital 'A' to represent the presence of the recessive gene on the allele, and a lowercase 'a' to represent the absence of the gene. 

There are 4 different combinations possible. The first and easiest, is visual to visual, or homozygous to homozygous, or in a Punnett square: AA x AA. Each offspring has a 100% chance of inheriting the recessive gene from both parents and all of them will show the trait.

The next combination is a visual to a het, or homozygous to heterozygous, or in the Punnett square: AA x Aa. This combination passes at least one copy of the recessive gene to all of the offspring. Any offspring that do not display albinism in this pairing will be "100% het for Albino".

Next up is "het to het" or in the punnett squale: Aa x Aa. This particular square is the one that shows how we get to 66% pos het for some animals. The 4 squares each represent a 1/4 chance that the offspring will inherit the set of genes contained within. Since albinism is recessive, the only square that would be visible albino is the red on, 'AA'. Since we are able to see that that 1/4th of the babies has the gene, we can divide the remaining animals into 3 groups. 2 out of those 3 groups will be het for albino, or 2/3 which equals 66%. This also happens to be where the female hognose in the header image sits. She is 66% pos het for Axanthic so we are unsure if she carries the gene or not. The male that we paired her to is 100% het axanthic so if she is actually het axanthic then 25% of the eggs should contain axanthic western hognose!

The final combination is het to normal, or Aa x aa. In this pairing, one parent is known to carry one copy of the recessive gene while the other does not. This results in 50% pos het offspring because the parent carrying the gene will pass it to 50% of the offspring but there is no visible way to tell which ones.

This can get a lot more interesting when you start to add more genes into the mix. For instance, the Punnett square below is for one of our future Kenyan sand boa pairings where 1 dominant and 2 recessive traits are present. The father here is a stripe het for both paradox albino and anery. The mother is a paradox albino het for anery. The combination of albino and anery is called 'snow', and the goal is to make stripe paradox snow babies. There is a 6.3% chance of succeeding.
The possibilities from this pair are:

• Stripe Paradox Snow: 6.3 %
• Stripe Paradox Albino Het Anery: 12.5 %
• Stripe Het Paradox Albino Anery: 6.3 %
• Stripe Het Paradox Albino Het Anery: 12.5 %
• Paradox Snow: 6.3 %
• Paradox Albino Het Anery: 12.5 %
• Het Paradox Albino Anery: 6.3 %
• Het Paradox Albino Het Anery: 12.5 %
• Stripe Paradox Albino: 6.3 %
• Stripe Het Paradox Albino : 6.3 %
• Paradox Albino: 6.3 %
• Het Paradox Albino : 6.3 %

Knowing how to use a Punnett square is not necessary for breeding but it does make learning about the possibilities easier. When you know the probability of an offspring having a trait, it makes it that much easier to identify.