Introduction to Medaka Selective Breeding

Welcome to the World of Medaka Selective Breeding

"I want to create my own original strain"—more and more medaka breeders are harboring this dream each year. Medaka have a short generation time (capable of reproducing in 3-4 months) and high genetic diversity, making it possible for even amateurs to see results from selective breeding in a relatively short period.

This article explains the fundamentals of genetics underlying selective breeding, as well as practical methods for crossing and selecting medaka.

Genetics Basics: Mendel's Laws

The foundation of selective breeding is genetics. Characteristics in medaka such as body color and fin shape (traits) are determined by genes.

Dominant and Recessive Traits

When different alleles exist at the same locus, the trait that appears more readily is called "dominant," while the trait that appears less readily is called "recessive."

Example: Black body color (dominant) × white body color (recessive) → F1 generation is almost entirely black → When F1 fish are crossed together, white body color appears in approximately 1/4 of the F2 generation

Fixation (Homozygosity)

By repeatedly crossing fish from the same line (through sibling crosses or repeated selection), you can stabilize and fix specific traits.

Major Heritable Traits in Medaka

Body Color - Basic body colors such as white, black, yellow (Himedaka), blue, and brown - "Body luminescence" like that of Mikimoto medaka (the iridescent sheen on the back) - Multi-color combinations such as three-color and red-and-white patterns

Fin Shape - Long-fin, swallow-tail, and light-body types - Daruma (shortened body type)

Other - Light-body type (back-shining body form) - Special varieties such as milky and aurora medaka

How to Plan a Practical Breeding Program

1. Clarify your target phenotype: Be specific, such as "a three-color pattern with vivid red and white contrast"
2. Select breeding stock: Choose males and females with characteristics close to your goal
3. Observe the F1 generation: Record the phenotypes of the offspring obtained
4. Selection and culling: Keep individuals close to your goal, and separate or rehome the rest
5. Repeat through F2 and F3 generations: Traits gradually become fixed

Selection Tips

• Prioritize keeping individuals with clear body shape and body color
• Exclude individuals with deformities or poor health from selection
• Keep records (photos and notes) to manage your lines
• Running multiple lines in parallel helps distribute risk

Conclusion

Medaka selective breeding is a long-term project, but the joy of watching the young fish change generation by generation is truly special. By understanding the basics of genetics and gradually continuing selection toward your goal, you will eventually establish your own unique line. Work on it at your own pace and enjoy the process.