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A field guide to the language of morphs

Morph genetics in plain English.

What does “het” actually tell you? Why can a normal-looking hatchling still carry a trait? Start with one pair of alleles, then see where simple percentages stop being enough.

See a worked pairingOpen the calculator
Start here

One address. Two copies.

For the ordinary single-locus examples on this page, each parent passes on one of their two alleles at a particular position. This is a useful model for many documented colour and pattern traits, but it does not explain every morph.

01 / Address

Locus

A position in the genome. Think of it as the address we are studying.

02 / Version

Allele

One version of the DNA at that address. An animal ordinarily inherits one from each parent.

03 / Result

Genotype → phenotype

The pair of alleles is its genotype at that locus. The visible or measurable trait is its phenotype.

Definitions: NHGRI, “Allele”, “Genotype” and “Phenotype”. This is a teaching model for a diploid, autosomal locus. Sex-linked inheritance and complex variation need separate treatment.

The term you will see everywhere

Het means two different versions.

Heterozygous, shortened to “het”, means the animal has two different alleles at a particular locus. Homozygous means the two copies are the same. “Het” alone never tells you whether an animal looks different: that depends on the inheritance pattern.

For a simple recessive morph, a het has one usual allele and one morph allele. It can look typical yet pass the morph allele to young. Breeders often call this animal a carrier. For an incomplete dominant morph, the one-copy het may already be visibly different.

The letters below are placeholders for one locus. C is the usual allele and c the recessive morph allele in the worked example, not a universal code for any species.

CCTwo usual copies
CcHet: different copies
ccTwo morph copies

Amalia Dutra, PhD, NHGRI: “Heterozygous” · NHGRI: “Homozygous”.

How the two copies show

Four patterns people mix up.

Recessive

Two copies to show

A one-copy animal may look typical; two copies produce the defined morph. The royal python Clown trait is a documented recessive example.

Dominant

One copy can show

One copy affects the phenotype. When the one-copy and two-copy animals look much the same, appearance alone may not prove the dose.

Incomplete dominance

One and two copies differ

The heterozygote has its own visible appearance, distinct from both the usual and two-copy forms. Mojave royal python is a familiar example in breeder terminology.

Codominance

Both versions show separately

Two different alleles are expressed together in the heterozygote, as in the textbook human AB blood group. In scientific usage this differs from an intermediate one-copy appearance.

A naming trap: reptile listings sometimes say “co-dom” for a morph better described as incomplete dominant. Read the described one-copy and two-copy phenotypes, rather than relying on the nickname. “Super” usually names a two-copy form in the trade; it says nothing by itself about health or viability. Check the specific morph and welfare register.

Principles: OpenStax, Biology 2e, §12.2 and Julie C. Sapp, NHGRI: “Codominance”. Reptile naming example: MorphMarket, “Understanding Genetics”. Clown association: Garcia-Elfring and colleagues, Pigment Cell & Melanoma Research (2025).

Worked example · one recessive locus

Het × het: what does 66% mean?

Suppose both parents are proven Cc for the same recessive trait. Each egg or sperm receives either C or c. Put those possibilities together and the four equally likely boxes give the odds for one hatchling.

Parent 1: C or c across the top. Parent 2: C or c down the side.
♀ × ♂Cc
CCCCc
cCccc

That is 25% CC, 50% Cc and 25% cc. If the recessive trait is fully expressed only in cc, three quarters look nonvisual. Of those nonvisual young, two of the three boxes are carriers: 2 ÷ 3 = 66.7% possible het. This is a conditional chance, not proof about a particular animal.

50% possible het comes from a different example: Cc × CC produces half Cc and half CC, all nonvisual under the simple model. Once parentage, test results or later offspring add information, the estimate should change.

Method: OpenStax, Biology 2e, Punnett square and probability. The possible-het convention is also explained by MorphMarket. Fractions and worked letters are YoGecko’s illustrative calculation.

Where the simple square stops

Polygenic is not polymorphic.

These words sound related but answer different questions. Neither is another name for a recessive carrier.

Polygenic

Several genes shape one trait

A colour intensity or pattern may vary across a range because several loci contribute. A tidy 25/50/25 single-locus prediction cannot describe it without an evidence-based model of those contributors. The environment can also affect some traits.

Lucia Hindorff, PhD, NHGRI →
Polymorphism

DNA variation exists in a population

In genomics it describes two or more forms of a DNA sequence among individuals; a SNP is one small kind. In population biology it can also describe several visible forms. Neither usage means “polygenic” or simply “het”.

Chris Gunter, PhD, NHGRI → OpenStax on population variation →
Line-bred

Selected over generations

Breeders select relatives or members of a line for a desired look. A line name alone does not identify a single causal allele. Repeated close breeding can increase homozygosity and expose harmful recessive variants, so pedigree and welfare history matter.

Purdue University animal breeding glossary →

Allelic complex

Two named variants may be different alleles at the same locus. An offspring can inherit one of each, and their combined appearance may have its own name. Count these as alternatives at one address, not as two independent genes. The calculator now models corn snake Amel/Ultra and Motley/Stripe, and a small part of the royal python BEL complex. MorphMarket’s examples explain the broader idea.

Linkage

Nearby loci on a chromosome can travel together more often than a simple independent-odds calculation predicts. Recombination can separate them. Charles Rotimi, PhD, NHGRI explains the principle.

Penetrance and expressivity

Penetrance asks whether a genotype shows its associated trait at all; expressivity asks how strongly it appears. A mild appearance does not establish that a linked health risk is absent. US National Library of Medicine explains both terms.

Sex-linked traits and sex determination

Inheritance depends on the species’ sex-determining biology. Do not paste a human X/Y example onto a gecko, snake or dragon. YoGecko’s current calculator treats its included loci as autosomal and does not model sex linkage.

What a label can prove

Record the evidence, and its limits.

Appearance, a claimed parentage and a laboratory result are different kinds of evidence. Ask which trait, which line, which parents and whether a DNA test has been validated for that variant and species. “Possible het” is a probability statement until stronger evidence arrives.

Autumn R. Brown and colleagues (2022) associated distinct variants in TYR, OCA2 and TYRP1 with three royal python colour morphs using shed skins. This is a useful example of how a trade name can be tested against DNA rather than assumed to describe a gene.

A healthy animal and an ethical pairing come before a novel colour. Review the YoGecko morph welfare register and the current REPTA policy before a real breeding decision.

Use the calculator with context

What YoGecko models

  • Fifty-one curated loci across seven species, including three allele groups.
  • Parent genotypes and uncertain carrier probabilities, when entered.
  • Independent assortment across the selected loci.

What it cannot establish

  • The identity or health of an individual animal.
  • Other allelic complexes, linked loci, polygenic lines or sex linkage.
  • Fertility, embryo viability, clutch size or the live-hatchling ratio.
Open the genetics lab
Further reading · checked 28 September 2026

Who these explanations draw on