Basic Breeding Part One: Using a Punnett Square
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By Jennifer Greene
Ball Pythons have some of the most diverse and beautiful combinations of mutations that affect their color and pattern. In the last 10 years, the number of genetically inherited traits that we have discovered in ball pythons is easily several dozen of single, simple traits, with the combination of those traits easily numbering into the hundreds. For the average person just beginning to scratch the surface of ball python breeding, learning about all the morphs and mutations, and all the fancy names for them, can seem extremely daunting. When you own morphs and are trying to create new ones, or just figuring out what you could potentially hatch out when you breed together animals carrying different traits, it can seem nearly impossible to memorize all the possible combinations and outcomes. Fortunately, you don't need to memorize the hundreds of combinations; instead, you can use a formula called a Punnett Square to predict your chances of hatching out specific types of offspring. Using a punnett square properly will enable you to figure out potential offspring for any possible combination of traits.
In this article, it's my goal to help you understand how to use a simple punnett square. To learn how to combine two, three, or more traits in a punnett square, I highly recommend picking up The Complete Ball Python, which has two excellent chapters on punnett squares that will help you out. In addition, search online for a free tutorial on genetics to help you out, or even consider enrolling in a basic biology course for a more thorough understanding. Next month's article will cover basic breeding principles such as inbreeding, line breeding, outcrossing, and their relevance in reptile breeding programs over the short and long term.
First, let's talk about recessive traits. These traits are only visible when an animal has two copies of the gene, one from each parent. Some examples of recessive traits are clowns, piebalds, ghosts, and the various types of albinos. Understanding how to predict your clutches is fairly easy - use that punnett square! It's a fairly simple method of determining probability per egg of what could hatch out.
Below I've drawn a simple square - for any single recessive trait, this is all you need to do to determine your chances of hatching out each baby. For punnett squares, the use of capital and lowercase letters indicates which gene is dominant over another. In recessive mutations, the normal type is going to be dominant over the recessive trait, so the capital letter A is going to mean the normal gene, while the lower case a means the recessive trait: albino for this example. Each trait has two copies of the gene, so when writing out the genes of an animal, you'll always use two letters: a het albino would be Aa, and an albino would be aa, and a normal would be AA.
Along the top of your square, put one of the parents. In our example, we'll have a het albino breed with an albino. Up top, I'll have the het albino parent (which gender they are doesn't matter). Along the side goes another parent (again, gender doesn't matter for this), and this one will be the albino. Each box gets 1 letter.
When you carry down the letters to fill in the box, you'll see that you get two possible outcomes: Aa, het albinos, and aa, albinos. Since one of the parents was an albino, all normal looking babies are going to be het albinos. You might be asking about what happens when you breed two het albinos together. Well, this is what that punnett square would look like:
You see that there are now 3 types of outcomes. AA, or completely normal babies, Aa, or het albino babies, and aa, or albino babies. The albino babies will be easy to pick out when they hatch, but what about the normal and het babies? They both have at least one copy of the normal gene, which means they will look totally normal. This situation is how possible hets are made. Since there is no visible difference between a normal ball python and a het albino ball python, instead many breeders will sell the offspring at a discounted price compared to guaranteed hets, and call them 66% hets. The 66% refers to the probability of each normal looking baby being a het, it's a short hand way of saying that the normal babies have a 66% chance of being het for albino. Buying these kinds of hets is a kind of calculated gamble, but can be a great way to score some hets for a discounted price. There are hets sold as smaller percents, such as 50% hets and 33% hets, and the same kind of short hand applies. Any time there is a percent in front of the word het, what should be referred to is the percent chance of that animal being het.
Ball Pythons are one of the most rewarding species to breed for fancy morphs because so many of the morphs are visible in the first generation. These morphs are referred to as codominant in the reptile hobby, although technically the term is inaccurate (it's been noted that the correct term should be incomplete dominant). Codominant morphs have a “super" form, which is when an animal has two copies of the trait. To show you in the form of a punnett square, writing out codominant traits is a little different than recessive. The capital letter in this case is the trait that is more dominant, so for a pastel, P refers to the pastel trait, while p is the normal gene. We'll write up a punnett square to describe breeding a pastel to a pastel.
Three potential combinations occur: normal babies, pp, more pastels, Pp, and super pastels, PP. Another term for super pastels is homozygous pastel, meaning that they have two copies of the pastel gene. A simpler definition for homozygous is same, meaning that the genes are the same, while heterozygous means mixed, or the genes are not the same. For some codominant traits, the homozygous form may look nothing like the heterozygous form. Lessers, Butters, and Mojaves are an example of this - they are all the heterozygous form of a blue eyed leucistic snake. Yellowbellies are the heterozygous form of the ivory ball python, and fires are the heterozygous form of the black eyed leucistic. Many codominant mutations have a homozygous (or super) form that looks like an extreme version of the heterozygous form, and when new mutations come out, discovering what the super expression looks like is one of the most exciting aspects of proving out the morph.
Fortunately for ball python breeders, the majority of ball python morphs are codominant, meaning that in the first generation of offspring you should see some babies that are visible morphs. This makes them extremely gratifying for the beginner, as you see results relatively quickly. There are also a handful of morphs that for all intents and purposes, are dominant traits, meaning that there is no super form. Dominant traits express the same regardless if the animal has one or two copies of the gene. Spider ball pythons and Pinstripe ball pythons could both be considered dominant, although popular opinion on spider ball pythons is still inconclusive as to whether or not a “super" form exists. Generally speaking, it is frowned upon to breed spider x spider or pinstripe x pinstripe, as it can result in weaker or misshapen offspring that often do not thrive. In addition, many do not see any real benefit to that breeding, as your chances for producing more morphs per clutch do not increase significantly. I'll create the punnett square for you to look at the pairing and the results. We'll do a pinstripe x pinstripe, with P indicating the pinstripe gene, and p indicating normal.
So, as you can see we get slightly increased odds of hatching out a pinstripe per egg. Instead of a 50% chance of each egg having a pinstripe in it (as would be the case when breeding a pinstripe to a normal), you have a 75% chance per egg of hatching out a pinstripe. However, that 75% actually breaks down into a 50% chance of a regular, or heterozygous pinstripe (only one copy of the pinstripe gene) and a 25% chance of a homozygous, or super, pinstripe. Now, pinstripes don't have a super form that looks different from the form with only one copy of the gene. This means that when you create a homozygous pinstripe, you will not be able to tell it apart from your heterozygous pinstripes.
Why does it matter if you can tell a homozygous pinstripe apart from a heterozygous pinstripe, you ask? Because when you raise up that baby homozygous pinstripe and breed it to anything else, it will always produce pinstripes! That makes it a valuable addition to a breeding project, although not visually more interesting than any other morph. Morphs that are genetically homozygous will always produce offspring that are morphs, which is why they generally fetch a higher price and can be considered more valuable to breeding projects. For a pet, there is no difference or benefit to getting a morph beyond appearance, so make your decision on a pet snake based on what you find the most visually appealing. When it comes to pet snakes, the genes behind their appearance are only as important as you want them to be.
There is one last type of morph that is available, and often quite a steal for a fancy looking pet snake, and that's the Ã“non-genetic" morphs. Labyrinths, Jungles (not to be confused with pastels, which are sometimes archaically referred to as pastel jungles), and a handful of other unique looking ball pythons are different in appearance than a normal ball python, but genetically they are no different. While many folks refer to these snakes as “non-genetic" morphs, that is a bit of a misnomer, as appearance is always to a certain extent controlled by the genes of the animal (thus, genetic). However, these morphs are not inheritable, or passed on from one generation to the next. If you were to breed a Jungle ball python to a normal, you would get entirely normal babies, with no increased likelihood of hatching more jungles. Occasionally, when temperatures fluctuate greatly during incubation, pattern or color can be affected: I personally have seen a clutch of extremely unique looking pastels with scrambled and reduced pattern that were the result of some accidental and extreme temperature fluctuations during incubation. They were very pretty, but their unique appearance was not a trait they were able to pass on to their offspring.
In closing, I just want to emphasize again the importance of selecting snakes to add to your collection that you enjoy and want to keep. If breeding, especially on a larger scale, is your goal, then consider taking a basic course on genetics (there are numerous free tutorials online). The punnett square is an extremely useful tool for understanding odds and probabilities for certain crosses, but remember that each punnett square is calculating your odds per egg, not per clutch, and does not tell you the guaranteed outcome of each breeding. It is a helpful tool to see possibilities, and not a fortune telling device.