Scale Gene: Metallic, Matt, Nacreous
Three main scale types: metallic (reflective, like gold or red), matt (no reflection), and nacreous (partial reflection, calico pattern).
Metallic x metallic = all metallic. Metallic x matt = all nacreous. Matt x matt = matt only. This is Mendelian co-dominance.
Color Change Gene
Most goldfish fry hatch dark olive-brown. The color-change gene activates at 2-6 months, revealing true adult color (red, orange, white, black).
Fish without the color-change gene stay wild-olive forever — these are rare and sometimes sold as "brown goldfish."
Calico Pattern
Calico requires nacreous scales + multiple color genes. The classic calico has orange, black, white, and blue (melanophores showing through).
Breeding two calicos gives only 25-30% calico offspring because it requires heterozygous nacreous scales plus color distribution.
Black, Panda, and Fading
Pure black fish (black moors) often fade to gold by age 3. True pandas keep black pattern by selective breeding from stable lines.
Fading is more common in bright lighting and warm water. Keep black fish in cooler water (18-20°C) with less light to delay fading.