Medaka Genetics & Variety Fixation — Breeding Selection Basics

Medaka (Japanese rice fish) breeding becomes more systematic when you understand genetics. The goal of variety fixation is to "consistently produce offspring with the same characteristics as the parents." This article covers the fundamentals of medaka genetics and selection methods for variety fixation.

Basics of Medaka Genetics

Medaka body color and traits are determined by combinations of multiple genes. The fundamental laws of genetics follow Mendel's laws taught in school.

Dominant and recessive traits: Each trait is controlled by two genes (alleles). If even one dominant allele is present, the dominant trait appears. The recessive trait only appears when both alleles are recessive (homozygous recessive).

For example, regarding the gene controlling black pigment in medaka, the normal type (b+) is dominant and the orange hi-medaka type (b) is recessive. A heterozygous b+b individual appears normal but carries the hidden recessive b allele.

Multiple gene combinations: Medaka body color involves not just one gene but multiple genes controlling black pigment, yellow pigment, iridescent pigment, and more. Their combinations produce the diversity of white medaka, blue medaka, amber medaka, and many other color varieties.

What Is Variety Fixation?

Variety fixation means repeatedly crossing individuals with a specific trait until the majority of offspring consistently express that trait. It can also be described as increasing the proportion of homozygous (carrying two identical alleles) individuals in the population.

Single-gene recessive traits (e.g., white medaka) are relatively easy to fix. Crossing homozygous recessive individuals produces offspring that all share the same trait.

However, many popular varieties (tricolor, lame/sparkle, body shine, etc.) involve multiple genes and are also influenced by environmental factors, requiring considerable time and generations for fixation.

Managing F1 Through F3 Generations

Let us follow the fixation process generation by generation.

P generation (parent generation): Select a male and female with the ideal traits for pairing. Same-bloodline individuals are preferable, but consider the downsides of inbreeding (increased deformity rates, reduced vigor).