Ball Python Genetics: Pt. 1
This is going to be a fairly rough walkthrough. It probably requires you to have a basic knowledge of genetics and Punnett squares. I will do my best to clarify ball python genetics in both their frequently used terms as well as the genetically correct terms. If you need some clarification, please let me know.
If you see something that is incorrect please send me an ask instead of reblogging! It is incredibly tedious to continually reblog this long post to answer concerns. You followers and my followers will thank you.
Many people use terms incorrectly within the ball python industry. Many people will describe a morph as a gene, when they really mean allele, or codominance, when they really mean incomplete dominance.
In genetics, a gene is a series of code that is responsible for expressing a genetic characteristic. For example, scale color would be one gene. Alleles, on the other hand, are variants of that gene. For example, ball python scale color would be the gene, and Mojave would be one allele. Another allele would be lesser. Usually within the ball python hobby, people use the terms interchangeably, which is incorrect.
Another one of these cases resides in the use of codominance. Traditionally, codominance means the alleles are equally expressed. This is different from incomplete dominance, which is intermediate expression of both alleles. Try to think of it this way, if a codominant red flower and a codominant white flower were to reproduce, the offspring would be both red and white. If an incomplete dominant red flower reproduced with an incomplete dominant white flower, the offspring would be pink. Codominance expresses both phenotypes, or the visual manifestation of the characteristic, whereas incomplete dominance meets somewhere in the middle.
For the remainder of this write-up, I will be using the terms interchangeably, although that is incorrect.
Other useful terms include:
Phenotype: The physical expression of a gene
Genotype: The codes for the gene. They usually appear as two letters, either lowercase or uppercase to signify the characteristics. Sometimes differing alleles will be dictated by a +.
Homozygous: has two of the same form of an allele (for example PP or pp)
Heterozygous: has two different forms of the allele (for example Pp)
When speaking of genes and forming Punnett squares, I like to think that genes are being plugged in. Each individual has two slots for each gene. If we go back to the flower comparison again, imagine that we are looking at the petal color slots of two flowers.
Here we have a red individual whose petal color slots are filled with PP (purple boxes). P means the color expressed will be red. The white individual has slots filled with pp (green boxes). p means the expressed color will be white. Now each parent will pass down one of their slots to fill the slot of their offspring. The red parent can pass down a P or a P. The white parent can pass down a p or a p. The resulting offspring would have slots filled with both a P and a p because each parent passed one slot down.
Now let’s apply that to different gene forms.
Complete Dominance Inheritance
Complete dominant genes (usually just called “dominant”) are always expressed in the phenotype. They are kind of like show offs. If they appear in the coding, they stand out over the other gene. Pinstripe and spider are both examples of dominant genes.
Example: Here are the parents.
Both are pinstripes. Both of the parents are heterozygous pinstripes which means they display the pinstripe gene (A), but also carry the wild-type gene (A+).
When paired, they will produce the following:
Each offspring receives one allele from each parent as depicted by the colored blocks. Try to figure out which individuals will be pinstripe and which will be wild-type in appearance.
75% of offspring will have the pinstripe phenotype and 25% will have wild-type appearance (remember phenotype is the physical outcome and genotype is the code that made that happen). Genotype probabilities are: 25% to produce a homozygous Pinstripe (AA), 50% to produce a heterozygous pinstripe that carries the wild-type gene (AA+), and 25% chance to produce a homozygous wild-type (A+A+).
Dominant genes will still have the same phenotype, therefore their homozygous and heterozygous individuals will look the same. Keep this in mind as this will change depending on the type of inheritance.
Recessive genes hide behind other genes and only show when they are paired up with a letter that looks like them. Recessives only show if the genotype is homozygous for the recessive gene, in other words the are two slots must be filled with the recessive gene for it to have a visual effect. Recessive genes are often expressed with lower case letters. Pied, axanthic, hypo, albino, clown, and genetic stripe are all examples of recessive genes.
Let’s start out by pairing a homozygous Pied (bb) to a wild-type (BB).
The only available genes to pass on are either a b or a b from the pied or a B or B from the normal, therefore all offspring will be visually wild-type, but heterozygous for pied.
The resulting offspring will display the dominant phenotype of wild-type but it hides the recessive gene of pied behind it.
Percent Het for a gene:
Breeders will sell the animals who carry recessive genes as “x% het.” This describes the probability of a pairing to create an animal actually carrying the recessive gene. The options you may encounter are: 100% het, 66% het, or 50% het and each depends on the pairing that was done to produce the offspring. As shown above, if a visual recessive (bb in the example above) is bred to anything, the offspring will all carry the gene. That recessive will pass down to 100% of the offspring.
66% hets are produced by breeding two heterozygous individuals. Let’s take two pied offspring from the previous example and breed them together.
In the first boxes, both parents pass on the dominant wild-type trait, therefore the individual will not carry the recessive trait. The 2nd and 3rd boxes are the same, as one parent passes on the wild-type allele and the other passes on the recessive pied allele. Finally, both parents pass on their recessive allele.
The odds are: 25% of the offspring will be homozygous dominant (BB) and phenotypically wild-type, thusly they will not carry the gene. 50% of the offspring will be heterozygous and phenotypically wild-type, and 25% will be homozygous recessive and phenotypically pied.
*Remember pheno= physical appearance and geno= code to make that appearance
The het percentage comes from the three phenotypically wild-type snakes. ¼ of the offspring produced will be visual pied, so there is no guesswork involved. You know those ones have the pied gene. As for the remaining 75%, you would be unable to tell who carries the pied gene and who does not. ⅔ of the remaining animals could carry the pied gene, while ⅓ would not. So, the probability of a snake from this pairing producing a wild-type visual that is also carrying the recessive gene is 66%. (⅔ * ⅓ = 2/9 = .66)
Finally, if you breed a heterozygous (Bb) to a homozygous dominant (BB):
You get a good ole 50/50 split. The first parent does not carry the recessive gene, so there is no possibility of getting a homozygous recessive from this pairing (and therefore no visual pied), but the recessive gene could have still been passed down by the heterozygous parent.
Here’s a Punnett square if you prefer seeing it that way:
The het parent, being the only one with a recessive gene, can then pass it on to 50% of the offspring. That is where the term 50% het comes from.
Alright, do you think you have a handle on all of that? If you do, we can move onto our next topics. This gets particularly tricky.
So tricky, in fact, that it’s going to take me some extra time to finish the write up. Keep an eye out for part 2!
As always, if you have any questions, my ask box is open!!