# Quantitative traits and heritability: why most good traits are hard to fix

> Yield, potency, terpene totals, vigour and flowering time are controlled by many genes plus environment. Heritability (the share of variation due to genes in a given population and setting) predicts how much of a selected plant's advantage passes to its offspring: response = heritability x selection differential. Cannabinoid ratios are highly heritable; absolute THC % and yield much less so.

- Canonical: https://genetixbay.com/knowledge/breeding/quantitative-traits-heritability.html
- Category: Breeding · Level: Breeder
- Evidence: Established · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Any stage · 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

**What is heritability in plant breeding?** Heritability in plant breeding is the share of the variation you see in a trait that is genetic, in a specific population and environment. Broad-sense heritability (all genetic variance divided by total variance) matters for clone selection. Narrow-sense heritability (additive variance only) matters for seed breeding, because only additive effects reliably pass to offspring.

**Why don't the offspring of my best plant perform as well?** Offspring of your best cannabis plant usually regress towards the average because much of that plant's edge came from environment, such as pot position or light, and from [dominance](https://genetixbay.com/knowledge/glossary/dominant.md) and gene interactions that don't breed true. With a yield heritability of 0.3, picking plants 100 g/m² above a 400 g/m² average predicts offspring around 430 g/m², not 500.

**Is [THC](https://genetixbay.com/knowledge/glossary/thc.md) percentage genetic or environmental?** THC percentage in cannabis is both genetic and environmental. The [THC:CBD ratio](https://genetixbay.com/knowledge/glossary/chemotype-ratio.md) is very highly heritable, but absolute THC percentage has only moderate heritability and is strongly affected by light, [harvest timing](https://genetixbay.com/knowledge/glossary/harvest-window.md) and the testing lab. Research found absolute [cannabinoid](https://genetixbay.com/knowledge/glossary/cannabinoid.md) amounts varied much more than ratios, so a 2-point difference between two labs proves nothing.

**What is the breeder's equation?** The breeder's equation is R = h² x S, where R is the response (how much the next generation's average moves), h² is narrow-sense heritability and S is the [selection differential](https://genetixbay.com/knowledge/glossary/selection-pressure.md), meaning how far the chosen parents sit above the population average. It can also be written R = i x h² x σ_P, using selection intensity: about 1.75 for the top 10%.

**How do I reduce environmental noise in a pheno hunt?** Reduce environmental noise in a cannabis pheno hunt by growing siblings side by side in identical pots, medium and feed, randomising positions because edge plants get different light. Score 2-4 clones of each candidate in a second run, use the same lab, harvest stage and top-cola sample for chemistry, and weigh and date every measurement.

**What is genotype by environment interaction?** Genotype by environment interaction is when cultivars rank differently in different conditions, for example cultivar A beating cultivar B in coco but not in soil. It is a separate part of total trait variation from genetics and environment alone. It means a keeper selected in one setup may not stay the best in another, so test candidates across conditions.

**Short answer.** A quantitative trait is one shaped by many genes of small effect plus the environment, so it varies continuously (grams, percent, days). Heritability tells you how much of the variation you see is genetic in *your* population and *your* room. When heritability is low, the best-looking plant is often just the luckiest, and its offspring regress towards the average.

## What's happening

Observed variation splits into parts [1]:

**V_P = V_G + V_E + V_GxE**, and **V_G = V_A + V_D + V_I**

- V_A, **additive**: the part that reliably passes to offspring.
- V_D, **dominance** and V_I, **epistasis** (gene interactions): show in an individual or [F1](https://genetixbay.com/knowledge/glossary/f1.md) but do not breed true.
- V_E, **environment**: pot position, light, watering, pests.
- V_GxE, **genotype by environment**: cultivar A beats B in coco but not in soil.

Two heritabilities:

- **Broad-sense (H²) = V_G / V_P.** Useful for clone selection, because a clone copies the whole [genotype](https://genetixbay.com/knowledge/glossary/genotype.md).
- **Narrow-sense (h²) = V_A / V_P.** Useful for seed breeding, because only additive effects reliably pass on.

## The breeder's equation

**R = h² x S** [1]

- S (selection differential) = mean of the plants you keep minus the mean of the population.
- R (response) = how much the next generation's mean moves.

Example: a population averages 400 g/m². You keep plants averaging 500 g/m² (S = 100). If h² for yield in that setting is 0.3, expect next generation's mean around 430 g/m², not 500.

## What we know for cannabis

| Trait | Heritability (indicative) | Notes |
|---|---|---|
| THC:CBD ratio ([chemotype](https://genetixbay.com/knowledge/glossary/chemotype.md)) | Very high | Largely one inheritance unit; see [chemotype inheritance](https://genetixbay.com/knowledge/breeding/chemotype-inheritance.md) |
| Cannabinoid composition ratios | High; stable within a genotype | Absolute amounts varied much more than ratios [2] |
| Absolute THC % | Moderate | Strongly affected by light, harvest timing and lab [2][4][5] |
| Flowering time | Moderate to high, with major genes | Cultivar origin predicted flowering in hemp [4] |
| Yield | Low to moderate | Strongly environment driven [5] |
| [Terpene profile](https://genetixbay.com/knowledge/glossary/terpene-profile.md) (which terpenes) | Moderate to high | Tandem terpene synthase genes; see strain-name article |
| Morphology and phenology | Lower than cannabinoid traits in [2] | |

Naim-Feil et al. (2023) concluded that cannabinoid composition should respond more strongly to selection than morphological and phenological traits [2] (*Supported*). Published cannabis heritability values come from small, single-site studies; treat specific numbers as tentative.

Lab numbers add noise too: THC results for similar products varied systematically between testing labs in Washington State [3]. A 2-point THC difference between two plants tested at two labs is not evidence of anything.

## How to raise your effective heritability

You cannot change the genes' heritability by wishing, but you can cut V_E:

1. **Grow siblings side by side in the same pots, medium and feed.** Randomise positions; edge plants get different light.
2. **Replicate.** Score clones of each candidate (2-4 each) in a second run before choosing.
3. **Use the same lab, same harvest stage, same sample position** ([top cola](https://genetixbay.com/knowledge/glossary/cola.md)) for chemistry.
4. **Measure, don't guess.** Grams dry per plant, days to finish, weighed and dated in the journal.
5. **[Progeny test](https://genetixbay.com/knowledge/glossary/progeny-test.md).** The best evidence a plant is genetically superior is how its offspring perform.

## Numbers that matter

| Concept | Formula | Use |
|---|---|---|
| Broad-sense heritability | H² = V_G / V_P | Clone selection |
| Narrow-sense heritability | h² = V_A / V_P | Seed lines |
| Response to selection | R = h² x S | Predict gain |
| Selection intensity | i (e.g. top 10% ≈ 1.75; top 20% ≈ 1.40) | R = i x h² x σ_P |

Selection intensities are standard normal-distribution values [1].

## Common mistakes

- Picking the "keeper" from one run of one plant per seed. You are partly selecting the luckiest pot.
- Treating THC % as a fixed property of a cut. It isn't [2][3].
- Selecting on many traits at once with tiny populations. Gains per trait shrink fast.

## Evidence

- Variance partitioning and the breeder's equation: *Established* [1].
- Cannabinoid ratios more heritable than absolute content and morphology: *Supported* [2].
- Lab-to-lab THC variation: *Supported* [3].
- Specific cannabis heritability values: *Contested* (small studies).

## Sources

1. Suza W, Lamkey K (eds). 2023. Quantitative Genetics for Plant Breeding. Iowa State University Digital Press. https://iastate.pressbooks.pub/quantitativegenetics/
2. 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
3. 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
4. Zhang M, Anderson SL, Brym ZT, Pearson BJ. 2021. Photoperiodic flowering response of essential oil, grain, and fiber hemp (Cannabis sativa L.) cultivars. Frontiers in Plant Science 12:694153. https://doi.org/10.3389/fpls.2021.694153
5. Rodriguez-Morrison V, Llewellyn D, Zheng Y. 2021. Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Frontiers in Plant Science 12:646020. https://doi.org/10.3389/fpls.2021.646020