Bristlenose Pleco
Genetics Calculator
Select parent morphs and instantly predict offspring ratios — albino, longfin, het carriers and more. “Het” is short for heterozygous: a fish that carries one hidden copy of a recessive gene. It looks like a normal common fish, but it can still pass that gene to its fry — so two hets paired together can produce visibly albino or longfin babies. Built for serious bristlenose pleco breeders.
New to bristlenose? Read the full care & breeding guide →
How the Bristlenose Genetics Calculator works
Written by an active bristlenose breeder working toward albino long-fin lines. Here is the genetics behind the calculator, how to read the results, and how to plan your pairings.
The two traits this calculator models
Bristlenose plecos (Ancistrus) have two well-documented colour and fin mutations that breeders work with: albinism (recessive) and long-fin (recessive). Both are autosomal — they are not sex-linked — and they sort independently, which means a single pairing can produce up to nine distinct phenotype combinations. The calculator models both loci using standard Mendelian inheritance and shows you the percentage chance of each outcome.
Reading the genetics notation
Each fish has two copies of each gene. We use uppercase for the wild-type (dominant) allele and lowercase for the mutant (recessive) allele:
Colour locus: AA = common (brown), Aa = het albino (looks common, carries the gene), aa = albino.
Fin locus: LL = normal fin, Ll = het longfin (looks normal, carries the gene), ll = longfin.
A fish that is AaLl is "double het" — it looks like a common normal-fin bristlenose but carries both recessive genes. Pair two double hets and you get a 1-in-16 chance of albino longfin offspring. The calculator works all of this out for you.
How to use the calculator
Step 1: set the male parent. Pick the colour (common, het albino, or albino) and the fin type (normal, het longfin, or longfin). If you do not know a fish's het status, you can still model it — use the visible phenotype only and the calculator will show the range of possible outcomes.
Step 2: set the female parent the same way.
Step 3: read the results. The calculator shows every possible offspring morph as a percentage card, sorted from most likely to least likely. Click "Show Punnett Details" to see the full 4×4 Punnett square with genotypes — useful when you are planning a multi-generation line and need to track carriers.
Planning pairings for a target morph
If you are working toward albino long-fins (as I am), the most efficient pairing is albino longfin × albino longfin — 100% of offspring will be albino longfin. But that requires you to already have both. More realistically, you pair an albino longfin with a double het (AaLl), which gives you 25% albino longfin and 75% double het. The 75% look common but carry the genes — keep them for the next generation. The calculator's "Hidden Carriers" tip tells you what percentage of fry will be hets so you can plan your grow-out accordingly.
Real-world caveats
Spawn size matters. A typical bristlenose clutch is 20–150 eggs. Small clutches may not reflect the theoretical ratios — if you only get 12 fry from a pairing that should produce 25% albino longfin, you might get zero albinos by chance. Larger clutches converge on the predicted ratios.
Fry identification takes time. Longfin fry show extended fins at 3–4 weeks. Albinism is visible from hatching. Do not cull or sell fry before 4 weeks — you cannot reliably identify long-fin carriers before then.
Sex ratios vary. Bristlenose sex determination is not purely genetic — temperature and conditions influence it. Do not assume a 50/50 split. If you need a specific sex for a future pairing, raise extra fry.
Frequently asked questions
What does "het" mean?
"Het" is short for heterozygous — the fish carries one copy of the recessive gene and one copy of the dominant gene. It looks like the common form but can pass the recessive gene to its offspring. Het fish are the backbone of any breeding line because they let you produce recessive morphs without losing genetic diversity.
Can I tell if a fish is het by looking at it?
No — het fish look identical to homozygous dominant fish. The only way to confirm het status is by test breeding: pair the unknown fish with a homozygous recessive fish. If any recessive offspring appear, the unknown is het. If you get 20+ fry with zero recessives, it is likely homozygous dominant (but never 100% certain — see the spawn size caveat above).
Why are my albino long-fin numbers lower than the calculator predicts?
Usually because one parent was sold as "het" but is actually homozygous dominant. Buy breeding stock from a reputable breeder who can show you the parentage — not from a pet shop. The other common cause is a small spawn size skewing ratios. Track your spawns in a spreadsheet over multiple clutches and the numbers will converge.
Should I cull the common fry from an albino pairing?
No. "Common" fry from an albino × het pairing are likely double hets — they carry both recessive genes and are valuable for your line. Sell them as hets (clearly labelled) to other breeders, or hold them back for future pairings. Culling healthy fish because they are not the target morph is bad practice and bad genetics — you narrow your gene pool for no reason.
How do I condition breeding pairs?
Feed protein-rich foods for two weeks before pairing — bloodworm, repashy, algae wafers, and fresh veg like zucchini and spinach. Keep temperature at 24–26°C. Provide a cave or PVC tube (3–4 cm diameter, 10–15 cm long). Do a 50% water change with slightly cooler water to trigger spawning. Full details in our bristlenose breeding guide.