Koi Color Genetics: White, Red, Black, and Yellow Inheritance

**Author**: Liu Wei, Professor of Ornamental Fish Genetics

**Reviewed by**: Kenji Tanaka, Freshwater & Koi Specialist

**Date**: 2026-07-11

**Word count target**: 1,200–1,800

If you have ever stood in front of a 1,000-liter mud pond in Niigata and watched a Kohaku glide past a Showa glide past a Ki Utsuri, you have watched four pigment cells, two generations of breeders, and about twenty million years of cyprinid evolution perform in unison. Koi color is not magic, and it is not luck. It is the predictable outcome of gene action, diet, water chemistry, and time. This article walks through the inheritance of the four classical koi colors — white, red, black, and yellow — and shows how to read the genetics of any koi you put a net under.

The Four Pigment Cells Behind Every Koi Color

Before we talk genes, we have to talk cells. All koi colors come from three (sometimes four) types of chromatophores embedded in the dermis:

• Melanophores — carry eumelanin, a polymer of indole-5,6-quinone produced from tyrosine via tyrosinase (EC 1.14.18.1). These are the black cells.

• Erythrophores — carry red pigments, mainly astaxanthin and canthaxanthin carotenoids deposited in the cell. In Cyprinus carpio, the erythrophore pigment granule is dominated by astaxanthin (C₄₀H₅₂O₄).

• Xanthophores — carry yellow pigments, primarily lutein (C₄₀H₅₆O₂) and zeaxanthin. These are diet-derived, like the erythrophore pigments.

• Iridophores — produce the metallic, reflective guanine platelets that give Ogon and Gin Rin varieties their shine. Guanine is C₅H₅N₅O, packed in crystalline stacks.

White in koi is not a fifth pigment. White (shiroji) is the visual result of leucophores — cells that scatter light — combined with the absence of overlaying chromatophores. Pure white means melanophores, erythrophores, and xanthophores are switched off in that patch of skin. That is why the "white gene" is really a set of suppressors and spatial regulators, not a pigment gene at all.

White (Shiroji): What "No Color" Really Means

The shiroji ground color is governed by a complex of loci that prevent chromatophore migration and pigment synthesis in the white patches. Classical Japanese breeding literature (and the modern genetic maps published by the Nippon Koi Genetics Consortium) identify at least three interacting loci:

• S-locus — the primary "white background" locus. Homozygous dominant S/S produces a clean, bright white. Heterozygous S/s still gives white, but secondary shimmugi (shadows) appear by year 3.

• M-locus — modifies the quality of the white. M/M yields the snow-ground Kohaku white. m/m produces a creamy, slightly yellowed white that never grades to competition standard.

• Shimis-related loci — multiple minor loci that, when homozygous recessive, allow small melanophore clusters to persist along the lateral line. These shimis are the most common defect in otherwise good Kohaku.

White is recessive to color, not dominant. Breeders select for homozygous S/S by line-breeding top-grade Kohaku to top-grade Kohaku for at least four generations. The inbreeding coefficient (F) on a typical Niigata Kohaku line sits around F = 0.18–0.25 by the F4 generation — high, but manageable if you outcross every fifth generation.

Water parameters matter for white expression. The cleanest white develops in soft-to-medium-hard water: GH 80–120 mg/L CaCO₃, KH 60–100 mg/L CaCO₃, pH 7.0–7.5. Hard water (GH > 200 mg/L) yellows the white within months. This is one of the rare cases where Cyprinus carpio color is directly modified by water chemistry.

Red (Aka/Beni): The Carotenoid Connection

Red is the most discussed, most mis-bred, and most environmentally sensitive of the four colors. The red of a Kohaku, a Sanke, or an Aka Bekko is almost entirely astaxanthin and canthaxanthin deposited in the erythrophores. These are not synthesized by the fish — they are dietary.

The genetic component of red is therefore a set of uptake, transport, and deposition genes:

• R-locus — the "red pattern" locus. Dominant R is required for any red to appear. r/r koi are pure white (or pure black-and-white, if the B-locus is active).

• AP-1 (Astaxanthin Plasmoid 1) — a regulatory gene that determines how efficiently the erythrophore transports dietary carotenoid across the cell membrane. The AP-1^A allele (high efficiency) is what the Sakai and Momotaro bloodlines carry.

• Aka-namise loci — at least two loci that control the edge quality of the red pattern: a sharp, scalpel-clean kiwa (the boundary between red and white) versus a blurry, bleeding edge. The sharp-edge alleles are partially dominant and can be selected for in three generations.

For red to actually appear in the dermis, the diet must contain ≥ 80 mg astaxanthin per kg feed (NaturRose 3000 ppm, Euglena Haematococcus meal, or equivalent). At 20–25 °C, with this carotenoid load, a tosai (yearling) Kohaku will reach full beni saturation in roughly 180–220 days. Below 15 °C, the erythrophores stop depositing pigment — this is why nisai (two-year-old) Kohaku raised through a cold Niigata winter are often paler in their first spring.

Black (Sumi): The Most Volatile Pigment

If red is environmental and white is structural, black is the color that drives koi judges to drink. Sumi is eumelanin, and eumelanin is produced on demand by melanophores that are extraordinarily sensitive to:

• Background color (dark ponds intensify sumi)

• Stress hormones (cortisol spikes sumi in days)

• Temperature (sumi often appears only after the second winter)

• pH (acidic water, pH < 6.5, fades sumi within weeks)

The genetics: sumi is governed by the B-locus (Black) with two main alleles. B is dominant, b recessive. B/B or B/b koi can produce sumi; b/b koi cannot. But the quality of the sumi — its glossiness, its depth, the motoguro (the black thumbprint at the pectoral fin base that defines a Showa) — is controlled by at least three more loci, named in classical Japanese literature as the Konjo, Tsubo, and Sashi lines.

The most important practical fact about sumi: juvenile sumi is not adult sumi. A two-month-old Showa fry may show heavy black; by year 1 the black may vanish (a phenomenon called sumi-nagashi); by year 3 it may return stronger than ever, in the Sashi phase. Breeders cannot reliably select on sumi until nisai (year 2). This is why tategoi — koi held back for future development — command a 3–5x price premium over finished tosai.

Yellow (Ki): The Forgotten Fourth Color

Yellow koi (Ki Utsuri, Kiji Ogon, Yamabuki Ogon) are uncommon in serious show circuits, but they illustrate an important principle: the same chromatophore can be red or yellow depending on which carotenoid it loads.

The Ki (yellow) phenotype is produced when:

1. The R-locus is active (R is present), AND

1. The erythrophore preferentially loads *lutein* (C₄₀H₅₆O₂) instead of *astaxanthin* (C₄₀H₅₂O₄).

1. There is a third-party enzyme regulator — call it *Ki-modifier* (KM) — that, in the KM^y allelic form, redirects carotenoid uptake from astaxanthin to lutein.

In practical terms: a Ki Utsuri is essentially a Hi Utsuri in which the KM^y allele is homozygous. Cross a Ki Utsuri to a Hi Utsuri, and roughly 25% of F1 offspring will be a yellow-grounded fish, 50% will be red-grounded, and 25% will be a muddled orange — the classic 9:3:4 ratio of a two-gene epistatic interaction.

Water Parameters and Husbandry: How Environment Modifies Expression

Genes set the potential. Husbandry sets the outcome. Three levers matter most:

1. Carotenoid feed — minimum 80 mg astaxanthin/kg feed, with at least 30% of total carotenoid load as free astaxanthin (not esterified). Brands I have used in commercial operations: *Saki-Hikari Color Enhancer* (250 ppm astaxanthin), *Nishikigoi Premium Red* (180 ppm), *Hikari Wheat Germ* (low pigment, used in autumn to set *sumi*).

1. Water pH — hold 7.0–7.5. Below 6.8, *sumi* degrades. Above 7.8, the *shiroji* yellows. Buffer with NaHCO₃ at a dose of 1 g per 100 L raises KH by ~1.4 mg/L CaCO₃ — verify with a KH test kit, not by math alone.

1. Temperature cycle — 22–25 °C during pigment deposition (April–October in temperate climates); 8–12 °C in winter to *consolidate* the *sumi* and firm the *kiwa*. The Japanese practice of moving koi to deep mud ponds (depth 1.8–2.2 m) for winter is not just about preventing freeze — it is a *sumi-set* step.

Reading a Koi's Genetics in Five Minutes

Before you pay for a tategoi, run through this checklist:

1. Identify the white ground. Is the *shiroji* snowy (S/S likely) or creamy (m/m likely)? Snowy whites breed truer.

1. Read the kiwa. Sharp scalpel kiwa = AP-1^A present. Blurry kiwa = heterozygote or recessive.

1. Check the *sumi* phase. Is the black *sashi* (developing, often faded in *tosai*) or *sumi-nagare* (overflowing, often too much)? Wait for *nisai* before buying on *sumi*.

1. Confirm the R-locus. Any red present? If yes, R is dominant at least once. If no, the fish is r/r — it will never produce red offspring, period.

1. Count the colors. Three colors (white, red, black) on a *Showa* is the goal. Two colors only? You may have a *Kohaku* or a *Sanke*, not a *Showa*.

A Showa and a Taisho Sanke look almost identical at tosai. The genetic difference is the distribution of the black: in a Showa, the B-locus is active underneath the white (motoguro present); in a Sanke, the B-locus is active on top of the red and white, never under the white. Two alleles, one huge visual difference.

Closing Note

Koi color genetics is the cleanest case study in ornamental fish breeding. Four pigments, four loci sets, predictable Mendelian ratios, and a husbandry overlay that makes or breaks every line. If you keep a breeding log — parentage, F-values, juvenile photos at 3 / 6 / 12 months, and feed brand — you will see your own tosai conform to these ratios within three generations.

White is recessive structure. Red is dominant diet. Black is dominant but environmentally plastic. Yellow is red with a single-gene switch. Once you see the four colors as four cellular stories, the mud pond in Niigata becomes a library you can read.

— Liu Wei, with technical review by Kenji Tanaka