Lethal super forms and sex-linked genes in reptiles
Two exceptions that make a standard Punnett square give the wrong answer — one about which animals hatch, one about which sex they are.
When two copies are not viable
A handful of well-known genes are lethal in their homozygous form. An embryo inheriting two copies does not develop, so the animals that hatch are the survivors of a clutch that has already lost a quarter of its members.
The odds among the hatchlings are therefore not the odds a Punnett square gives. The dead class is removed and the remainder renormalised, which turns the familiar 25 / 50 / 25 into roughly two-thirds and one-third.
- Super Spider
- not viable
- Spider, among hatchlings
- ≈67%
- Normal, among hatchlings
- ≈33%
Which genes, and what it costs
In HerpSync's shipped dataset, Spider and Champagne in ball pythons and Lilly White in crested geckos are flagged as homozygous-lethal. Breeding two of the same together is a real decision: a quarter of the clutch is lost before it hatches, and there is no version of the pairing where that is not true.
Spider carries a second, separate welfare consideration — the neurological 'wobble' associated with the gene, which varies in severity between individuals and is not something breeding practice has eliminated. That is a judgement each breeder makes, but it should be made knowing what it is rather than discovered afterwards.
Sex-linked genes and the ZW system
Snakes and many other reptiles use a ZW sex-determination system, the mirror image of the mammalian XY one: males carry two Z chromosomes and females carry one Z and one W. A gene on the Z chromosome is therefore inherited differently by sons and daughters.
Banana in ball pythons is the well-known example, and it is why the hobby talks about male-maker and female-maker lines. Depending on which parent carries the gene, a pairing can throw Bananas that are overwhelmingly one sex.
A calculator that treats Banana as an ordinary co-dominant gene produces a plausible-looking number that is wrong about the thing breeders most want to know.
Which parent is which matters
For every other gene it makes no difference which animal you call the sire. For a sex-linked gene it decides the answer, because the sire contributes a Z to every offspring while the dam contributes a Z to her sons and a W to her daughters.
HerpSync splits sex-linked results by offspring sex and says plainly when a pairing's sexes are not set, rather than quietly assuming one.
Temperature is not genetics, but it is inheritance-adjacent
Some species — leopard geckos among them — have temperature-dependent sex determination, where incubation temperature shifts the sex ratio of the clutch. It is not genetic and no calculator can predict it, but it is a real breeding decision made at the same moment.
It belongs in your incubation records rather than in a genetics engine, which is where HerpSync keeps it.
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
Modes of inheritance
Four words do most of the work in reptile genetics, and three of them are routinely used to mean the wrong thing.
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.
Common questions
Why does Spider × Spider not give 25% Super Spider?
Because the homozygous Super Spider is not viable — those embryos do not develop. The clutch that hatches is missing that quarter, so among the surviving hatchlings roughly two-thirds are Spider and one-third are normal.
Which reptile morphs are homozygous lethal?
In HerpSync's curated dataset: Spider and Champagne in ball pythons, and Lilly White in crested geckos. Breeding two of the same gene together loses roughly a quarter of the clutch before hatching.
What is a male-maker Banana?
Banana sits on the Z chromosome, so which parent carries it determines the sex ratio of the Banana offspring. Lines are described as male-makers or female-makers depending on which pattern they produce — a consequence of sex-linked inheritance rather than anything unusual about the individual animal.
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