Recessive, dominant, co-dominant and polygenic — reptile inheritance explained
Four words do most of the work in reptile genetics, and three of them are routinely used to mean the wrong thing.
Recessive — two copies to show
A recessive gene is only visible when an animal has two copies of it, one from each parent. An animal with a single copy looks completely ordinary and is called a het — short for heterozygous — and carries the gene invisibly.
This is why recessive projects take time. You cannot see what you are holding, so the first generation of a recessive project produces animals that look like nothing at all and are worth something only because you know their parents.
Most corn snake morphs work this way, as do Albino, Piebald and Clown in ball pythons.
- Visual Albino
- 100%
- Normal-looking
- 0%
Co-dominant — one copy shows, two make a super
A co-dominant gene changes the animal's appearance with a single copy, and two copies produce a distinctly different, usually more extreme animal with its own name — the super form. Pastel and Super Pastel, Mojave and Super Mojave, Anaconda and Superconda.
This is the mode that makes a project visible from generation one, which is why so much of the ball python and boa market is built on it. You can see what you are holding, and the super is a real target rather than a hidden carrier.
Strictly, most of these are incomplete dominance rather than true co-dominance. The hobby settled on 'co-dom' decades ago and it is not worth fighting; what matters is that one copy shows and two copies look different again.
- Super Pastel
- 25%
- Pastel
- 50%
- Normal
- 25%
Dominant — one copy shows, two look the same
A dominant gene is visible with one copy, and an animal with two copies looks no different from an animal with one. There is no super form to breed towards, which changes what a project is for: you are producing the morph, not working up to something rarer.
Pinstripe and Woma in ball pythons behave this way, as does Tessera in corn snakes.
Two dominant genes in the shipped ball python dataset — Spider and Champagne — are a special case, because the homozygous form is not merely indistinguishable but non-viable. That has its own guide.
Polygenic — selected, not switched on
A polygenic trait is produced by many genes acting together, and intensified by choosing the best animals to breed from over several generations. Tangerine in leopard geckos, Harlequin in crested geckos, and every established trait in gargoyle geckos are polygenic.
There is no single gene to inherit, which means there is no honest percentage to give. A calculator that produces a number for a polygenic trait is making it up.
That does not make the trait less real or less valuable — line-bred animals are often the most expensive in the hobby. It makes the record keeping matter more, because the only evidence that a line is improving is what your own pairings actually produced.
Why the mode is the first thing to establish
Almost every expensive mistake in reptile breeding traces back to assuming the wrong mode. Paying a premium for a het that was actually a possible het. Expecting a super from a dominant gene that has none. Buying two line-bred animals and expecting Mendelian odds.
HerpSync stores the mode on the gene itself, alongside its allelic locus and any lethal or sex-linked flag, so the calculator applies the right rules without you having to remember which species behaves how.
Check it in the calculator
Everything on this page is modelled by the free morph calculator — no account, nothing to install. Pick two parents' genes and it shows the per-gene odds and the full combination table.
Keep reading
Hets & possible hets
The most misunderstood label in the hobby, and the one most likely to be attached to money.
Allelic series & compound hets
Two morphs that occupy the same position on the chromosome behave in a way ordinary Punnett squares get quietly wrong.
Lethal supers & sex-linked genes
Two exceptions that make a standard Punnett square give the wrong answer — one about which animals hatch, one about which sex they are.
Common questions
What is the difference between co-dominant and incomplete dominant?
In strict genetics, co-dominance means both alleles are expressed separately, and incomplete dominance means the heterozygote is intermediate. Most reptile 'co-dom' morphs are technically incomplete dominance. The hobby uses co-dominant for both, and the practical meaning is what matters: one copy shows, two copies produce a distinct super form.
Is a dominant gene better than a recessive one?
Neither is better — they suit different projects. A dominant or co-dominant gene shows in the first generation, so results are visible immediately. A recessive gene hides in hets, which makes a project slower but means the visual animals stay comparatively scarce.
Can a trait be partly polygenic?
In practice, yes — a single gene can set the baseline while selective breeding intensifies the result, which is why two animals with the same named gene can look noticeably different. HerpSync predicts the Mendelian part and records the rest rather than pretending the whole thing is calculable.
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