Allelic morphs and compound hets — when two morphs share one locus

Genes live at positions on a chromosome called loci. Most of the time each named morph is a mutation at a locus of its own, and they assort independently — inheriting one tells you nothing about the other.

Sometimes two or more named morphs are different mutations at the same locus. They are allelic, and an animal has only two slots at that position. That single fact changes the arithmetic.

An animal cannot be homozygous for two allelic genes at once, because there is nowhere to put the second pair. It also cannot inherit them independently — a gamete carries exactly one allele from that locus, so a parent carrying both passes on one or the other, never both and never neither.

That means a pairing that a naive calculator predicts will produce a double-visual animal can produce none at all, forever.

• In the shipped ball python data: Lesser and Butter, Cinnamon and Black Pastel and Mocha, Phantom and Mystic, Toffee and Candy
• In corn snakes: Amelanistic and Ultra, and separately Motley and Stripe

An animal carrying one copy of each of two allelic recessives has no normal allele at that locus at all — so it is not a het for anything. It shows a phenotype, usually intermediate between the two homozygotes, and the hobby normally gives it its own name.

The clearest example is the corn snake Ultramel: one Amelanistic allele and one Ultra allele, producing an animal that looks like its own morph and is often sold as one. It is a compound het, and it will never breed like a het for either gene alone.

The best-known allelic group in the hobby is the ball python BEL complex, where combining two alleles from the group produces a blue-eyed leucistic animal rather than either parent morph. Lesser and Butter are the pair carried in HerpSync's dataset.

Anyone who has planned a BEL project on a generic calculator has probably seen it produce a number that cannot happen. Modelling the locus rather than the gene is what fixes that.

The engine works per locus rather than per gene. Genes that share a locus are grouped, gametes carry one allele from it, and the resulting allele pair determines the phenotype — including compound combinations that have their own names.

The allelic locus is a property of the gene in the dataset, so this is applied automatically. You do not have to know that Cinnamon and Black Pastel are allelic for the odds to come out right, though the species pages will tell you.

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.

Open the morph calculator