# Selection and pheno hunting: sample sizes and scoring rubrics

> A pheno hunt is a selection experiment: grow enough seeds to have a real chance of finding what you want, clone every candidate before flowering, score each plant on a fixed weighted rubric, and re-grow the top few as clones before you call a keeper. The plants needed follow 1 - (1 - p)^n: a 1-in-10 type needs 29 plants for a 95% chance of seeing at least one. Sex stability is a gate: any plant that herms is culled, whatever else it scores.

- Canonical: https://genetixbay.com/knowledge/breeding/selection-pheno-hunting.html
- Category: Breeding · Level: Advanced
- Evidence: Supported · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Veg, Flower, Harvest, Cure · Systems: Any system
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**How many seeds should I pop for a pheno hunt?** How many seeds to pop depends on how rare the target type is. For a 95% chance of finding at least one plant, a 1-in-4 type needs 11 females, 1 in 10 needs 29, 1 in 20 needs 59 and 1 in 100 needs 299. Double those for regular seed to allow for males, and add [germination](https://genetixbay.com/knowledge/glossary/germination.md) losses. Fewer than 10 females is a sampling.

**How do I pick a keeper?** Pick a cannabis keeper by writing a brief before popping seeds, cloning every seedling, scoring all plants on the same weighted rubric, shortlisting the top 10-20%, then re-running 2-4 clones of each shortlisted plant together. Most 'keepers' fall out in that second run. Cull any plant showing [pollen sacs](https://genetixbay.com/knowledge/glossary/anther.md) or nanners, however good it smells.

**What should I score in a pheno hunt?** Score cannabis plants 1-5 on vigour and health, structure, pest and mould resistance, flowering time, dry yield, resin and trim, blind-rated dry aroma, lab chemistry from the same lab and stage, and blind smoke reports, each multiplied by a weight fixed before scoring. Sex stability is a gate, not a score: any intersex plant is culled.

**Should I clone before flowering?** Yes, clone every cannabis seedling before flipping to flower, taking two cuts each and labelling them with the parent's ID. Without clones you can lose the winner at harvest, since a flowered seed plant is hard to keep. Discard the clones of rejected plants later, and use the clones of shortlisted plants for the confirmation run.

**How many times should I run a cut before deciding?** Run a cannabis cut at least twice before deciding it's a keeper: once in the seed hunt, then a second run with 2-4 clones of each shortlisted candidate grown side by side. A single run of one plant mostly selects the luckiest pot, because environmental noise can make a plant look better than its genes deserve.

**Short answer.** Decide what you are looking for before you pop a seed, grow enough plants to have a fair chance of finding it, clone every candidate, score every plant the same way, and confirm the winners in a second run. One run of one plant is a lottery ticket, not a selection.

> **Breeder's standard.** Sex stability is a gate, not a score. Any plant that shows pollen sacs or nanners during the hunt is out, along with its clones, however good it smells. Don't use it as a mother, a reversal donor or a pollen parent. Record the week and conditions, and report the line's [herm rate](https://genetixbay.com/knowledge/glossary/herm-threshold.md). Cannabis is a dioecious species; a herm in a hunt is a defect in that [seed lot](https://genetixbay.com/knowledge/glossary/seed-run.md), and the [light leak](https://genetixbay.com/knowledge/glossary/light-leak.md) that exposed it did you a favour.

## What's happening

In a [segregating](https://genetixbay.com/knowledge/glossary/segregation.md) seed lot (F2, polyhybrid, landrace), each seed is a different genotype. You are sampling a distribution. Two forces fight you:

1. **Rarity.** The combination you want may be 1 in 10 or 1 in 100.
2. **Environmental noise.** A plant in the best corner looks better than its genes deserve [1].

Good hunting raises your odds against the first and filters out the second.

## How many plants?

Probability of finding at least one plant of a type with frequency p in n plants: **P = 1 - (1 - p)^n**, so **n = ln(1 - P) / ln(1 - p)** [1].

| Frequency of target type | 80% chance | 90% chance | 95% chance | 99% chance |
|---|---|---|---|---|
| 1 in 4 (25%) | 6 | 9 | 11 | 17 |
| 1 in 10 (10%) | 16 | 22 | 29 | 44 |
| 1 in 20 (5%) | 32 | 45 | 59 | 90 |
| 1 in 50 (2%) | 80 | 114 | 149 | 228 |
| 1 in 100 (1%) | 161 | 230 | 299 | 459 |

Add germination losses and males (regular seed: double it). Practical rule: fewer than 10 females is a sampling, not a hunt.

## Before you start

- **Write the brief.** Three to five must-have traits and the weighted score below. Changing criteria mid-hunt makes you pick favourites.
- **Mint a GB-IND per seedling** and label pots with it.
- **Uniform conditions.** Same [pot size](https://genetixbay.com/knowledge/glossary/pot-size.md), medium, feed; rotate positions weekly.
- **Clone everyone** before flipping (2 cuts each). Discard clones of rejected plants later.

## Scoring rubric (score 1-5 each, multiply by weight)

| Trait | Weight | How to score | When |
|---|---|---|---|
| Vigour and health | 2 | Growth rate, leaf colour, no [deficiencies](https://genetixbay.com/knowledge/glossary/nutrient-deficiency.md) under standard feed | Veg |
| Structure | 1 | Internode spacing, branching, support needed | Late veg / week 3 flower |
| Sex stability | 3 (gate) | Any pollen sacs or nanners = cull plant and clones; note week and any trigger | Whole flower |
| Pest and mould resistance | 2 | [Powdery mildew](https://genetixbay.com/knowledge/glossary/powdery-mildew.md), botrytis incidence, mite tolerance | Whole run |
| Flowering time | 1 | Days from flip to harvest-ready | Harvest |
| Yield | 2 | Dry grams per plant (or per m²) | After dry |
| Resin and trim | 1 | Trichome coverage, leaf-to-calyx ratio | Late flower |
| Aroma (dry) | 2 | Intensity, distinctness; blind rated by 2-3 people | After cure |
| Chemistry | 2 | Lab cannabinoid and [terpene panel](https://genetixbay.com/knowledge/glossary/terpene-test.md), same lab, same stage | After dry |
| Smoke / effect report | 1 | Blind panel notes; reported, not promised | After cure |

Gate traits (sex stability, serious mould) remove a plant regardless of score. Weights are a *Grower practice* template; adjust to your brief, but fix them before scoring. Structured descriptors in DUS test guidelines are a good model for consistent scoring language [5].

## Step by step

1. **Germinate** the full lot together; record germination rate (it is QC data for the batch).
2. **Veg cull** only for gross faults (weak, deformed, sick). Don't cull on looks too early.
3. **Clone** all survivors, label clones with the parent GB-IND.
4. **Flower and score** using the rubric, with dated photos at weeks 3, 6 and harvest.
5. **Shortlist** the top 10-20% by total score.
6. **Re-run** shortlisted clones together (2-4 each). This is where most "keepers" fall out.
7. **Select** the keeper(s). Record why. For breeding, also keep the best males or note which females pass traits to offspring (progeny testing).

## Selecting males

You can't smoke a male. Use:
- vigour, structure, resistance and sex stability (no [female flowers](https://genetixbay.com/knowledge/glossary/pistillate-flower.md); a male that makes female flowers is intersex and is culled);
- stem rub aroma (*Grower practice*, weak evidence);
- **progeny testing**: cross each candidate to the same female and compare offspring. This is the only strong evidence of a male's breeding value.

## Common mistakes

- Popping 5 seeds and calling the best one "the keeper".
- Not cloning before flower, then losing the winner at harvest.
- Scoring THC from one sample at one lab and treating it as fixed [4].
- Letting a strong-smelling but intersex plant through "because the herm was stress". Stress reveals susceptible genetics; sex stability is a gate, not a bonus.

## Evidence

- Sample-size probability and selection theory: *Established* [1][2].
- [Cannabinoid ratios](https://genetixbay.com/knowledge/glossary/chemotype-ratio.md) more selectable than absolute amounts: *Supported* [3].
- Lab variability in potency: *Supported* [4].
- Rubric weights and male stem-rub: *Grower practice*.
- Sex stability as a hard gate (cull herm-prone plants, never breed from them): GenetixBay breeder's standard; see [diagnosing hermaphrodites](https://genetixbay.com/knowledge/diagnosis/hermaphrodites.md).

## Sources

1. Suza W, Lamkey K (eds). 2023. Quantitative Genetics for Plant Breeding. Iowa State University Digital Press. https://iastate.pressbooks.pub/quantitativegenetics/
2. Suza W, Lamkey K (eds). 2023. Quantitative Methods for Plant Breeding. Iowa State University Digital Press. https://iastate.pressbooks.pub/quantitativeplantbreeding/
3. Naim-Feil E, Elkins AC, Malmberg MM, et al. 2023. The Cannabis plant as a complex system: interrelationships between cannabinoid compositions, morphological, physiological and phenological traits. Plants 12(3):493. https://doi.org/10.3390/plants12030493
4. Jikomes N, Zoorob M. 2018. The cannabinoid content of legal cannabis in Washington State varies systematically across testing facilities and popular consumer products. Scientific Reports 8:4519. https://doi.org/10.1038/s41598-018-22755-2
5. UPOV. 2023. Test guidelines for hemp (Cannabis sativa L.), TG/276/1. https://www.upov.int/edocs/tgdocs/en/tg276_01.pdf