Methodology & Genetics
How What's the FURcast? calculates predictions, and the limits of those predictions.
How inheritance probabilities are calculated
For every locus the engine takes each parent's genotype, determines the alleles they can pass in their gametes (each at 50% for a heterozygote, 100% for a homozygote), and combines maternal and paternal alleles in a Punnett square to get the offspring genotype probabilities for that locus.
Assuming independent assortment, the per-locus distributions are multiplied together into a full joint genotype distribution. Each joint genotype is then translated into a visible phenotype using the dominance and epistasis rules below. Identical visible phenotypes produced by different underlying genotypes are grouped together and their probabilities summed, then sorted from highest to lowest.
Genotypes, phenotypes and loci
A genotype is the pair of alleles a dog carries at a locus (e.g. E/e). A phenotype is what you actually see (e.g. "black"). A locus is a gene location that controls one part of the coat.
Loci interact. The E locus can switch off eumelanin and hide K/A patterns. The K locus can mask the A locus. The B locus recolours eumelanin rather than adding a new colour. These interactions (epistasis) are applied in order before a phenotype is shown.
The loci modelled
Extension (E)
The Extension (E/MC1R) locus controls whether eumelanin (black/brown pigment) can be produced in the coat. Eᵐ (melanistic mask) is dominant and adds a dark eumelanin mask on the muzzle/face while still permitting normal eumelanin expression. eᴬ (hare-pied / husky marking) and eᴳ (domino / grizzle) partially restrict eumelanin and expand phaeomelanin markings. Recessive red (e/e) is epistatic to the K and A loci: an e/e dog cannot show eumelanin-based colours or patterns in the coat, so K/A/B patterns are evaluated only when E or Eᵐ permits eumelanin expression.
Alleles: Eᵐ > E > eᴬ > eᴳ > eʰ > e
Dominant Black (K)
The Dominant Black (K/CBD103) locus controls whether a dog has a solid eumelanin coat (Kᴮ), brindle striping (kᵇʳ), or allows the A-locus agouti pattern to show (kʸ/kʸ).
Alleles: Kᴮ > kᵇʳ > kʸ
Agouti (A)
The Agouti (A/ASIP) locus controls the distribution of eumelanin vs phaeomelanin across the coat — sable, wolf-sable, tan points, or recessive black. It is only visible when the K locus is kʸ/kʸ and the E locus permits eumelanin.
Alleles: aʸ > aʷ > aᵗ > a
Brown (B)
The Brown (B/TYRP1) locus converts black eumelanin to brown/liver/chocolate when b/b. It modifies eumelanin pigment everywhere it appears (coat, nose, lips, eye rims) but does not affect phaeomelanin (red/gold).
Alleles: B > b
White Spotting (S)
The White Spotting (S/MITF) locus controls the amount of white spotting. This is a simplified model: true Irish spotting is partly polygenic, so the categories here are an approximation of the expected degree of white from the S-locus genotype.
Alleles: S > sⁱ > sᵖ > sʷ
Merle (M)optional
The Merle (M/PMEL17) locus produces mottled patches of eumelanin. M/m = merle; m/m = non-merle; M/M = double merle, which is associated with significant health risks (hearing and vision defects). Merle only shows on eumelanin areas — it is not visible on a recessive-red (e/e) coat. This model treats merle as a simple M/m locus; real merle has variable expressivity and cryptic forms not captured here.
Alleles: M > m
Harlequin (H)optional
The Harlequin (H/PSMB7) locus modifies the merle pattern. H/_ turns the dilute grey areas of a merle coat to pure white, leaving torn irregular patches of solid eumelanin on a white base. It is only visible on dogs that are merle (M/m or M/M) and not recessive red (e/e). On a non-merle or recessive-red dog, H/_ has no visible effect. Recognised primarily in Great Danes.
Alleles: H > h
Furnishing (F)optional
The Furnishing (F/RSPO2) locus controls facial furnishings (beard/eyebrows) and wire coat. F/_ = furnished; f/f = unfurnished.
Alleles: F > f
Curl (Cu)optional
The Curl (Cu/KRT71) locus controls coat curl and shows incomplete dominance: +/+ produces a curly coat, +/- a wavy coat, and -/- a straight coat.
Alleles: + > -
Coat Length (FGF5)optional
The Coat Length (L/FGF5) locus controls hair length. The Lh allele is recessive, so two copies (Lh/Lh) are required for a long, silky coat; Sh/Sh and Sh/Lh produce a short coat. In certain breeds (e.g. Pembroke Welsh Corgi, French Bulldog) the long coat is described as 'fluffy'. Coat length as reported here is influenced by several genetic variants working together to promote long hair; this is a simplified single-locus model.
Alleles: Sh > Lh
Dilution (D)optional
The Dilution (D/MLPH) locus dilutes eumelanin pigment when d/d. Black becomes blue (grey); liver becomes lilac/isabella. Phaeomelanin (red/gold) is not affected, so a recessive-red dog still appears red, though its nose and eye rims may dilute.
Alleles: D > d
Cocoa (HPS3)optional
The Cocoa (HPS3) locus converts black eumelanin to dark brown pigment when co/co. Dogs with co/co produce dark brown pigment in both hair and skin instead of black. N/co dogs produce black pigment but can pass the co allele on to their puppies. Dogs that are co/co and also b/b at the B locus are generally a lighter brown than co/co dogs with B/_ at the B locus.
Alleles: N > co
Roan (R)optional
The Roan (R) locus produces a roan pattern — a fine mix of white and coloured hairs in the white areas of the coat. This is a simplified single-locus model; real roan expressivity varies and overlaps with ticking.
Alleles: R > r
Ticking (T)optional
The Ticking (T) locus produces small coloured flecks in the white areas of the coat. This is a simplified single-locus model; real ticking and roan are closely related and can overlap.
Alleles: T > t
Progressive Graying (G)optional
The Progressive Graying (G) locus causes the coat to lighten progressively with age, with a dose effect: g/g holds colour, G/g fades slowly (~40% lighter), G/G fades fast (~60% lighter). It affects all coat colours. This locus is not routinely DNA-tested, so it is optional.
Alleles: G > g
Saddle Tan (RALY)optional
The Saddle Tan (RALY) locus modifies the A-locus at (tan points) allele. N/_ causes the black to recede into a saddle with age; I/I keeps classic tan points. Dogs that do not express at are not influenced by this gene. Not routinely DNA-tested, so it is optional.
Alleles: N > I
Why optional loci may show as Unknown
Merle, Dilution, Roan, Ticking, Furnishing and Curl are optional. If a parent hasn't been tested, the predictor does not assume a negative genotype (e.g. it does not assume m/m for an untested Merle). Instead the trait is markedUnknown and is not used to split the predicted outcomes. Provide DNA results for both parents to refine the prediction.
Why the images are representations, not guarantees
Each distinct predicted phenotype gets one generated image built from the phenotype description (not from genetic notation). The image is a visual representation of that coat type — base colour, pigment, pattern, white spotting, merle, furnishing and curl where known. Real puppies vary in expression, and unknown traits cannot be depicted. Treat the images as illustrations of the predicted coat, not a promise of how an actual puppy will look.
Limitations of genetic prediction
- This is a simplified Mendelian model. Real canine coat genetics includes additional loci (intensity, cocoa, hair length, hairless and breed-specific modifiers) not modelled here, and roan/ticking expressivity overlaps in ways a single-locus model cannot fully capture.
- Irish spotting is partly polygenic; the S-locus categories are an approximation of the expected degree of white.
- Merle has variable expressivity and cryptic forms not captured by the simple M/m model. Double merle (M/M) carries serious health risks and triggers a warning.
- Curl shows incomplete dominance: Cu/Cu = curly, Cu/cu = wavy, cu/cu = straight.
- Probabilities describe the expected distribution across a large number of puppies; individual litters will vary.
- Always verify DNA results and consult an appropriately qualified canine geneticist or veterinarian before making breeding decisions.