FAQ

Sex-Linked Inheritance

Most morphs in the catalog are autosomal — they live on a regular chromosome, inherited the same way regardless of sex. A few sit on a sex chromosome, and that changes the math: sons and daughters get different outcomes from the same cross.

ZW vs XY — snakes are different

Mammals are XY: females are XX, males are XY. Birds, snakes, and many reptiles are ZW: females are ZW, males are ZZ. The naming flips, the inheritance pattern flips with it.

In a ZZ/ZW species:

  • A sire is ZZ — he has two Z's. He gives a Z to every offspring.
  • A dam is ZW — she has one Z and one W. She gives a Z to half her offspring (those become sons, ZZ) and a W to the other half (those become daughters, ZW).

A trait sitting on the Z chromosome is therefore inherited differently:

  • Sons (ZZ) inherit one Z from each parent.
  • Daughters (ZW) inherit a Z from the sire and a W from the dam — never a Z from the dam.

That asymmetry is what produces the famous "all my Banana babies are male" result.

Banana — the canonical Z-linked codominant

Banana (and its synonym Coral Glow) is a Z-linked codominant trait in ball pythons. The catalog flags it explicitly so the calculator builds per-sex offspring distributions when a Z-linked locus is in the cross.

Visual male Banana × normal female

Sire: ZᴮZ⁺ (one Banana Z, one wild Z, expressed visual). Dam: Z⁺W (no Banana).

  • Sons (ZZ): receive a Z from each parent. 50% get ZᴮZ⁺ (visual Banana), 50% get Z⁺Z⁺ (normal). So half the sons are Banana, half are normal.
  • Daughters (ZW): receive a Z from dad and a W from mom. Dad has Zᴮ and Z⁺, so 50% of daughters get ZᴮW (visual Banana), 50% get Z⁺W (normal). Half the daughters are Banana, half are normal.

So this cross is balanced — no male/female bias on the Banana side. The interesting case is the reverse.

Visual female Banana × normal male

Sire: Z⁺Z⁺ (no Banana). Dam: ZᴮW (visual Banana).

  • Sons (ZZ): receive a Z from each parent. Mom has Zᴮ (the only Z she carries, because the W isn't a Z). So every son inherits Banana from mom — 100% visual Banana sons. The other Z comes from dad, who has only wild-type, so sons are ZᴮZ⁺ — visual.
  • Daughters (ZW): receive a Z from dad and a W from mom. Dad has only Z⁺, so daughters are Z⁺W — all daughters are normal, none get Banana.

Result: all sons are visual Banana, all daughters are normal. That's the male-biased visual Banana clutch breeders see when they breed a visual female to a normal male.

Heterozygous female × normal male

Dam: ZᴮW if visual, but if she's a het — wait. There's a subtlety. ZW females only have one Z, so they can't be heterozygous in the autosomal sense. They are either expressing Banana (ZᴮW) or not (Z⁺W). For a Z-linked codominant like Banana, females can't carry it as a "het" the way an autosomal recessive het works.

The "het" idea reappears for Z-linked recessives (which the catalog doesn't currently model — Banana is codominant). In that case, sons can be ZᵃZ⁺ hets, daughters cannot. That's why visual Z-linked recessives often show up as a "must be male" trait until a same-locus female surfaces.

What this looks like in the calculator

When you cross two parents and any locus is flagged Z-linked, the results pane shows two distributions side by side: Sons and Daughters. The aggregate distribution (combining both) is also shown for sanity, but the per-sex view is what you'll use to plan a pairing or estimate sex ratios.

What this looks like when recording a hatching

The same split carries over into the Record Hatching dialog on a clutch:

  • Autosomal-only cross (m.f.u row). Outcomes are identical between sexes, so each predicted outcome gets three inputs in the same row: ♂ / ♀ / ?. This follows the m.f.u notation the herp community uses on classifieds and breeder lists — 1.2.4 reads as "1 male, 2 females, 4 unsexed". You can split a single outcome across all three columns in one save (e.g. Pastel: 1 ♂ + 2 ♀ + 4 ?), and animals are created with the matching sex per column. The ? column maps to unknown and is what most ball-python and python clutches use at hatch — most hatchlings are too young to sex reliably, and you fix individual sexes later.
  • Cross with a Z-linked locus (two columns). Outcomes differ between sexes, so the dialog shows Male offspring and Female offspring side by side. The column you enter the count in determines the sex of the hatchlings created — there is no ? column in this mode because the biology decides. A baby logged in the male column is created as male; the female column creates females.

This is why a Banana ball python clutch can hatch with hatchlings already correctly sexed in the system, while a corn-snake amelanistic clutch typically hatches as unknown until you sex them yourself (or partially sexed if you popped/probed). The app isn't being inconsistent — it's reflecting what the genetics actually tell us.

Other Z-linked traits

The catalog only models Banana / Coral Glow as Z-linked today. Other reptile trades (notably leopard gecko's Murphy "snow" complex, depending on how you classify it) may include Z-linked or W-linked claims; those aren't seeded yet because the per-locus evidence is still contested. If a trait you keep is Z-linked and missing, that is a clean addition to the catalog.