How fish genetics work

Every fish you see is the result of a coin toss its parents made for it. Once you know how the toss works, you can read a brood like a breeder: why two plain fish suddenly have a rare one, and what the odds are that the next pair will.

Two copies of every gene

A gene is the instruction for one trait: body colour, pattern, fin shape. Each version of a gene is called an allele. A guppy's colour gene, for example, can be grey, red, blue, yellow or magenta.

Every fish carries two copies of each gene, one from its mother and one from its father. When it breeds, it passes on one of its two, picked at random, like a coin toss. Its mate does the same. So each fry gets a fresh pair, one from each parent.

Which one shows

A fish has two copies, but you only see one of them. Some alleles show over others: we call the stronger one dominant and the hidden one recessive.

A fish with one grey and one magenta copy looks grey. The magenta is still there, carried silently, and it can be passed on. Magenta only shows when a fish has two magenta copies, because then there is nothing to hide it.

When a gene has more than two alleles, they form a ladder: the top one shows over all the others, the next one over all below it, and so on. The ones at the bottom are the recessives.

Carriers, and the surprise fry

A fish that hides a recessive allele is called a carrier. You can't tell a carrier from its looks. A grey guppy might be grey/grey, or it might be grey/magenta.

That is how two grey fish can have a magenta fry. If both parents carry magenta, each has a one-in-two chance of passing it on, and sometimes both do.

Working out the odds

Breeders use a small grid called a Punnett square: one parent's two alleles down the side, the other's across the top. Each box is one possible fry, and all four are equally likely.

Take two grey guppies that both carry magenta (grey/magenta × grey/magenta):

• grey + grey: a grey fry (1 in 4)
• grey + magenta: a grey fry that carries magenta (1 in 4)
• magenta + grey: a grey fry that carries magenta (1 in 4)
• magenta + magenta: a magenta fry (1 in 4)

So about one fry in four shows magenta, and two in three of the grey fry are carriers. In a brood of twelve you would expect about three magenta fry. It could be more or fewer, because every fry is its own coin toss.

If only one parent carries magenta, no fry can show it, but half of them will carry it. That is how a rare colour hides for generations and then turns up again.

Blends

Some alleles don't hide each other. When a fish carries two that share the same step on the ladder, it shows both at once, a blend. Breeders call this co-dominance. In Pondarium, a betta with one red and one blue copy shows both colours, and a koi with one kohaku and one showa copy shows a blend of the two. On the Genes page, a ◐ marks the alleles that blend.

Each gene on its own

Colour, pattern and fins are separate genes, and each is passed on with its own coin toss. A fish can get its colour from one side of the family and its fins from the other. That is why breeding for a fish with two rare traits takes longer: the odds multiply. A 1-in-4 colour and a 1-in-4 fin together is 1 in 16.

Mutations

Now and then a copy goes wrong, and a fry gets an allele neither parent carried. Most mutations are small, but they are also how brand-new colours and forms appear. They are rare, so you can't plan for them, but they are why every brood is worth a look.

In Pondarium

Pondarium's fish follow these rules. In the game:

• Logbook → Genes shows every gene of every species: each allele in order (each shows over the ones to its right), which are recessive, which blend, and how common each one is in the wild.
• A fish's Traits show what it looks like. What it carries hidden stays hidden until you find out, with a gene test or a lab sample.
• Before you pair two fish, the breeding preview shows the odds of each trait in the fry, worked out from what you know about the parents.
• The Gene Lab on Home has a new genetics puzzle every day: spot the carrier, find the father, or pick the pair most likely to give a rare look.