How the calculator works
Each gene is worked out on its own as a 2×2 Punnett square, just like a monohybrid cross. The calculator then combines the genes with the product rule, the same idea as the no-Punnett-square method for trihybrid crosses.
One gene needs extra care. At the agouti gene, the lethal yellow allele Ay is fatal with two copies, so any Ay/Ay box is crossed out. The remaining boxes are the pups that are born, which is why two yellow parents give a 2:1 ratio. The calculator also shows what share of embryos is lost.
The genes in the calculator
| Gene | Alleles (dominant first) | What it does |
|---|---|---|
| Agouti (a, the mouse version of ASIP) | Ay > A > a | Ay: yellow fur, lethal with two copies. A: agouti (wild-type, with banded hairs). a/a: solid black or brown. |
| Brown (Tyrp1) | B, b | b/b: brown instead of black pigment. Agouti mice with b/b are cinnamon. |
| Albino (Tyr) | C, c | c/c: no pigment at all; white fur, pink eyes. Hides every other gene. |
| Dilute (Myo5a) | D, d | d/d: pigment clumps inside each hair, so the coat looks paler. |
| Pink-eyed dilution (Oca2) | P, p | p/p: much less dark pigment in fur and eyes; pale coat, pink eyes. |
| Piebald spotting (Ednrb) | S, s | s/s: white patches. The amount of white varies a lot. |
What the calculator simplifies
- Fewer alleles than real mice have. The agouti gene has other alleles (such as white-bellied and black-and-tan), the albino gene has in-between alleles such as chinchilla and Himalayan, and many more coat genes exist.
- Independent genes. Each gene is treated as inherited independently. In real mice, albino (
c) and pink-eyed dilution (p) sit close together on chromosome 7, so they tend to be inherited together. Crosses using both will differ from the calculator's ratios. - Plain coat names. Shade, the amount of white and the exact look vary from mouse to mouse.
- Probabilities, not promises. A ⅔ chance means each pup that's born has a 2 in 3 chance, not that exactly two pups in every three will be yellow.
Frequently asked questions
Why do two yellow mice give 2 yellow : 1 non-yellow instead of 3:1?
Yellow mice are always Ay/a (or Ay/A), because Ay/Ay embryos die very early in development. A cross of two yellow mice still makes ¼ Ay/Ay, but those embryos never become pups. Among the pups that are born, ⅔ are yellow and ⅓ are not. See lethal yellow mice.
Why does albino hide every other gene?
Albino mice (c/c) have a tyrosinase enzyme that doesn’t work, so they can’t make any melanin. Without pigment, it doesn’t matter whether the other genes say black, brown, agouti or yellow: the fur is white and the eyes are pink. That masking is called epistasis. See white mouse genetics.
Are the coat names the same as mouse-fancy show names?
No. The calculator uses plain descriptions such as “Black, dilute (paler)”. Fancy-mouse clubs use their own names for many of these coats, and the names differ between clubs.
Why aren’t the results split into males and females?
All six genes in the calculator are on autosomes (not the X or Y chromosome), so sons and daughters have the same chances.
Can I use this to plan breeding?
It’s a teaching tool for Mendelian ratios, not a breeding guide. Real mice carry more alleles and modifier genes than any classroom model, and some of the coats here come with health effects (lethal yellow mice tend to become obese, for example).
Sources
- Wolfe HG, Coleman DL. “Pigmentation.” In Biology of the Laboratory Mouse, Chapter 21. The Jackson Laboratory, via Mouse Genome Informatics. informatics.jax.org/greenbook
- Michaud EJ, et al. “A molecular model for the genetic and phenotypic characteristics of the mouse lethal yellow (Ay) mutation.” PNAS 91, 2562–2566 (1994). doi:10.1073/pnas.91.7.2562
- Mouse Genome Informatics, Tyrc (albino) allele.