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Genes & perfection

Every koi is built from nine on/off genes, fifteen sliding genes, a set of pattern shapes and a few chance rolls. This page explains each one, how they pass from parent to fry, and what it takes to reach a near-perfect koi.

How a koi’s name is built

The on/off genes decide the variety. The pattern and traits decide its named form. Variants stack on the front in a fixed order.

Hirenaga+Gin Rin+Doitsu+Tancho+Named form+Variety

Drawing examples…

Gene reference

On/off genes have two alleles, one from each parent. Ladder genes have several alleles in a dominance order. Sliding genes run from 0 to 100%. Shape genes are pattern blocks copied from either parent. Chance traits are rolled for each fish.

Mendelian genes · decide the variety

Sliding genes · decide quality and look

Pattern shapes and chance traits

How inheritance works

  • On/off genes follow Mendel. Each fry takes one allele from each parent at random. A dominant allele shows if either copy has it; a recessive one (doitsu) only shows with two copies. Two carriers of a hidden allele give about one fry in four that shows it.
  • Sliding genes land near the parents’ average, pulled back towards a poor population average (heritability 0.6), plus random scatter and an occasional bigger jump. Breeding good with good is the only reliable way to great.
  • Unjudged sliders (growth rate, fin size, trail, Gin Rin amount, scale cover) work the same way but centre on 50%, so they drift towards average unless you keep selecting for them.
  • Pattern shapes are copied block by block from either parent with jitter. A good Kohaku pattern tends to reappear in its fry, but rarely exactly.
  • Care doesn’t change genes, only how they show. Fry raised in clean water on good food express their genes fully; neglected fish show up to 20% less. Breeding uses the genes, not the look.

Chasing perfection

Start from ordinary parents, keep the best female and male from every spawn, and breed them the next spring. This runs that loop with the game’s own breeding code, averaged over several lines.

Simulating…
  • Big broods matter. More fry per spawn means more chances of a lucky jump. Mud pond upgrades go from 10 fry to 40.
  • Care adds the last few points. Colour, edges, white and luster are judged as they show, and they only show fully in clean water on good food. Pattern shape isn’t affected by care, which is why the lines stay close.
  • Gains slow down near the top. Each generation is pulled back towards the population average, so the last few points take many years and some luck.
  • A perfect Tancho is the hardest prize. It needs top genes, a round centred crown (a pattern shape you can only select for), perfect care and the Tancho roll itself.

What koi are worth

Price is variety × size × quality × variants. Size and quality compound, so a fry is worth pennies and a big champion is worth thousands.

Selling price is 70% of value at mastery level 0, rising by 6% per level.

Value multipliers

Multipliers stack. A Gin Rin Tancho Kohaku with Diamond sparkle is worth 2.5 × 1.3 = 3.25 times an otherwise identical Kohaku.