Chapter 3 of 13 · Ball Python Breeding Guide

Genetics

Disclaimer: I'm not a veterinarian and I don't hold a degree in animal science or animal care. This is a guide I compiled and use for my own ball pythons. For a sick or egg-bound snake, a reptile vet is the authority.

The short version

  • Every baby gets one copy of each gene from mom and one from dad, picked at random.
  • A recessive gene, like Clown or Piebald, only shows with two copies. A snake with one hidden copy looks normal and is called a "het."
  • With an incomplete dominant gene, like Pastel, one copy shows. Two copies make a stronger look called the "super" form.
  • Het × het gives 1 visual baby in 4 on average. Each egg is a new roll, so a clutch can have none.
  • A "66% het" or "50% het" is a chance, not a sure thing. Only a DNA test or the snake's own babies can prove it.

The basics

Every ball python carries two copies of each gene, one from each parent, and passes one copy, chosen at random, to each baby. A "morph" is a snake with a visible mutation. How a mutation shows depends on its type.

A coiled Pastel ball python with bright yellow and black markings on wood shavings
A Pastel ball python: one copy of this incomplete dominant gene makes the yellow brighter than in a normal snake. Photo: Cody Campbell, CC BY-SA 3.0 via Wikimedia Commons.

Recessive. It only shows when a snake has two copies. A snake with one hidden copy looks normal and is called a het (heterozygous). Clown, Piebald, Albino, Hypo and Axanthic are recessive [1][2][3]. Some of these have now been traced to specific genes: Piebald to TFEC [4], Albino to TYR [5], Clown to MC1R [6] and VPI Axanthic to gch2 [7].

Incomplete dominant. One copy shows, and two copies make a different, usually stronger, look called the super form: Pastel and Super Pastel, for example [1][8]. Pastel, Enchi, Lesser, Mojave, Banana (Coral Glow), Cinnamon and Yellow Belly are listed this way [1].

Two Punnett squares for an incomplete dominant gene, using Pastel as the example. A purple dot stands for one Pastel copy and a blue dot for one normal copy. Left square, Pastel times normal: two boxes have one Pastel copy and are labelled Pastel, two boxes have two normal copies and are labelled normal. Result: half Pastel (50%), half normal (50%). Right square, Pastel times Pastel: one box has two Pastel copies and is labelled Super Pastel, two boxes are Pastel and one is normal. Result: 1 in 4 Super Pastel (25%), 2 in 4 Pastel (50%), 1 in 4 normal (25%). A note at the bottom says that with an incomplete dominant gene one copy shows, and two copies make a different, usually stronger look called the super form.
With an incomplete dominant gene like Pastel, one copy shows and two copies make the super form. Diagram: Amnesia Oddities. Tap the diagram to open it full size.

Dominant. One copy and two copies look the same. Pinstripe is the usual example [9].

A word on terms: hobby listings often use a different word for genes like Pastel. In genetics, a single copy that looks partway between normal and the two-copy form is incomplete dominance, and that's the term this guide uses [8].

Recessive crosses at a glance

ParentsBabies
visual × visualall visual
visual × hethalf visual, half het
visual × normalall het
het × het1 in 4 visual, 2 in 4 het, 1 in 4 normal
het × normalnone visual; half are het, but you can't tell which
Two Punnett squares for a recessive gene, using Clown as the example. A purple dot stands for one Clown copy and a blue dot for one normal copy. Left square, het Clown times het Clown, where both parents look normal: one box has two Clown copies and is labelled visual, two boxes have one of each and are labelled het, and one box has two normal copies and is labelled normal. Result: 1 in 4 visual (25%), 2 in 4 het (50%), 1 in 4 normal (25%). Right square, visual Clown times het Clown: two boxes are visual and two are het. Result: half visual (50%), half het (50%). A note at the bottom says a recessive gene only shows with two copies, a het has one hidden copy and looks normal, and each egg is a fresh roll of the dice.
Each parent passes one copy at random, so het × het gives 1 in 4 visual babies on average and visual × het gives half visual, half het. Diagram: Amnesia Oddities. Tap the diagram to open it full size.

Possible hets. From het × het, the normal-looking babies are each 2 in 3 (66%) likely to be het. From het × normal, each baby is a 50% possible het [10]. These percentages are chances for one animal, not a gene it carries.

A young albino ball python, white and yellow with pink-red eyes, coiled on a white towel
An Albino ball python makes no black pigment, so it is white and yellow with pinkish eyes; the gene is recessive, so it needs two copies to show. Photo: WingedWolfPsion, CC BY-SA 3.0 via Wikimedia Commons.

Each egg is a fresh roll. "1 in 4" is an average over many eggs. A clutch of six from het × het can easily give no visuals, or three.

Proving a het

  1. DNA test. Labs test shed skin for many recessive genes and report whether the snake carries none, one or two copies. Rare Genetics Inc. (RGI) offers single-gene tests and panels [11]. Not every gene has a test yet, so check the lab's list before you order [11].
  2. Test breeding. Breed the possible het to a visual. If the parent really is het, each baby has a 1 in 2 chance of being visual [10]. Any single visual baby proves it. Normal-looking babies make "het" less likely but never rule it out for sure. After 10 normal babies in a row, the chance the parent is het is about 1 in 1,000 for a 50% possible het, and about 1 in 500 for a 66% one [12].
Our plan: we won't sell a snake as het unless a DNA test or its own babies have proved it.

Genes that share a slot (allelic complexes)

Some mutations sit at the same place in the genome. A snake can carry only two of them in total, and two different ones together can make something new.

A pale yellow and tan ball python with an unusual broken pattern, lying in a display box
This snake carries two genes, Butter and Clown; Clown is recessive and changes the pattern into an unusual, broken-up design. Photo: crudmucosa, CC BY 2.0 via Flickr.
  • The BEL group (blue-eyed leucistic): Lesser, Butter, Mojave, Russo, Phantom, Mystic, Special and others. Two of these together make a white or near-white snake with blue eyes [13]. Butter and Lesser are treated as the same mutation; RGI tests them together [11][1].
  • Clown and Cryptic are different changes in the same gene (MC1R). One copy of each makes the "Crypton" [6].
  • Albino and Candy share a gene (TYR) [5]. Albino, Ultramel and a third albino type (an OCA2 deletion) sit on different genes, so crossing any two of those gives normal-looking babies [5].
  • The Spider group (Spider, Champagne, Woma, Hidden Gene Woma, Black Head, Spotnose and others) is covered in Genes to avoid [13].

Lines that don't mix

Two snakes can look the same and still carry different mutations. Axanthic is the clearest case. The VPI line is a change in one gene, gch2 [7]. Other Axanthic lines are not in that same slot. If you cross two different lines, the babies look normal, and each one carries one copy of each line [14]. RGI sells a separate test for the VPI line and for each of two other Axanthic lines [11]. Pick one line and stay with it.

When the simple rules break

  • Desert Ghost looks recessive, but lab work found at least two separate genes involved; a visual needs a copy at each [15]. The lab says treating it as a simple recessive still works well for planning, but at least one more gene seems to play a part, so results may not match a calculator [15]. RGI now tests for both main genes [11].
  • Banana / Coral Glow is linked to the sex chromosomes. Pythons have XY chromosomes like mammals [16]. A male can carry the gene in one of two places. If it's on his Y (which came from his father), he mostly makes Coral Glow sons. If it's on his X (which came from his mother), he mostly makes Coral Glow daughters. A female passes it to about half her babies of either sex. Occasional crossover between X and Y explains the odd "wrong" sex [17].
A coiled Banana ball python with bright yellow blotches on a pale purple-gray background
A female Banana (Coral Glow) ball python; this gene is linked to the sex chromosomes, so it does not always pass on the simple way. Photo: crudmucosa, CC BY 2.0 via Flickr.

Using a genetics calculator

Online genetics calculators list the possible babies from two parents [18]. They're a good check, with limits:

  • They assume one gene, simple inheritance. They don't handle Desert Ghost's extra genes, Banana's sex linkage, or complexes nobody has proven.
  • Some can't take "66% possible het" as an input; run the cross twice, once as het and once as not [19].
  • They say nothing about health. A cross can be genetically possible and still a bad idea.

Step by step

Planning a pairing:

  1. Write down every gene each parent has. Mark each het as proven or "possible."
  2. Check the lines. If either parent has Axanthic, make sure both are the same line.
  3. Check for genes that share a slot, like the BEL group, and read Genes to avoid.
  4. Use the table above, or a genetics calculator, to see the odds.
  5. If a parent is a 66% possible het, run the calculator twice: once as het and once as not.
  6. Remember the odds are averages. Each egg is a fresh roll.

Sources for this chapter

  1. MorphMarket Morphpedia gene pages: Clown, Hypo, Pastel, Enchi, Lesser, Mojave, Banana, Yellow Belly (hobby reference, not peer-reviewed)
  2. MorphMarket Support, Understanding genetics
  3. Morphpedia, Cryptic and Caramel Albino
  4. Piebaldism and chromatophore development in reptiles are linked to the tfec gene, Current Biology, NSF repository
  5. Brown, A. R. et al. (2022), albinism genes in ball pythons, PLOS ONE 17(10): e0276376, doi:10.1371/journal.pone.0276376
  6. García-Elfring, A. et al. (2025), Clown and Cryptic in MC1R, Pigment Cell & Melanoma Research 38(1): e13215, PDF
  7. García-Elfring, A. et al. (2025), VPI Axanthic and gch2, Animal Genetics 56: e70011, OMIA entry
  8. Northwestern University genetics glossary, Incomplete dominance
  9. MorphMarket Morphpedia, Pinstripe
  10. FurryCritter, Punnett squares (hobby site; the ratios are standard Mendelian genetics)
  11. Rare Genetics Inc. via MorphMarket, DNA testing
  12. Wikipedia, Bayes' theorem (general reference for the math: each normal baby from a het × visual cross halves the odds that the parent is het)
  13. MorphMarket Community, Allelic morphs (complexes) (community-maintained list)
  14. García-Elfring, A. et al. (2025), VPI Axanthic and gch2, including crosses between Axanthic lines, Animal Genetics 56: e70011, doi:10.1111/age.70011; open preprint at bioRxiv
  15. MorphMarket blog (RGI), Desert Ghost decoded
  16. Gamble, T. et al. (2017), XY sex chromosomes in a boa and a python, Current Biology 27(14): 2148–2153, doi:10.1016/j.cub.2017.06.010
  17. Mallery, C. S. and Carrillo, M. M. (2016), sex-linked Coral Glow in ball pythons, University of São Paulo herpetology journal, 15(1): 29–42, doi:10.11606/issn.2316-9079.v15i1p29-42
  18. MorphMarket Support, Genetic calculators
  19. MorphMarket Community, Possible-het input to the calculator (keeper forum, archived copy)

Last revised October 31, 2026.