# Inbreeding depression

> Inbreeding depression is the loss of vigour, fertility, yield or survival that follows mating between relatives, mainly because harmful recessive alleles become homozygous.

- Canonical: https://genetixbay.com/knowledge/glossary/inbreeding-depression.html
- Also written: inbreeding decline, inbred depression, loss of vigour from inbreeding
- Glossary category: Genetics and breeding · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

Inbreeding depression is reduced fitness in offspring of related parents: slower growth, smaller plants, poorer seed set, weak [germination](https://genetixbay.com/knowledge/glossary/germination.md), deformities or more susceptibility to stress. The main cause is that [inbreeding](https://genetixbay.com/knowledge/glossary/inbred-line.md) makes harmful [recessive alleles](https://genetixbay.com/knowledge/glossary/recessive.md) homozygous, so their effects show; loss of heterozygote advantage adds to it (Charlesworth and Willis 2009).

## In practice

Cannabis is naturally [outcrossing](https://genetixbay.com/knowledge/glossary/outcross.md), so it carries a load of hidden recessives and can suffer when inbred hard. Breeders see it after repeated selfing ([S1](https://genetixbay.com/knowledge/glossary/s1.md) on S1) or several sib generations with small populations. Signs: falling germination rates, runts, twisted leaves, lower yields, and in some lines unstable sex expression.

The defence is selection and population size. Cull hard each generation, keep many parents, and outcross when a line weakens. A good IBL is one where the breeder purged the defects along the way rather than inbreeding past them.

## Don't confuse with

- **Clone decline**: clones are not inbred and do not inbreed. A cut that weakens over years almost always has a pathogen (such as [hop latent viroid](https://genetixbay.com/knowledge/glossary/hop-latent-viroid.md)) or a care problem.
- **Genetic drift**: random loss of [alleles](https://genetixbay.com/knowledge/glossary/allele.md) in small populations; it often accompanies inbreeding but is a separate process.
- **Heterosis**: the opposite effect, from crossing unrelated lines.

## Related terms

[Inbred line](https://genetixbay.com/knowledge/glossary/inbred-line.md), [Selfing](https://genetixbay.com/knowledge/glossary/selfing.md), [Heterosis](https://genetixbay.com/knowledge/glossary/heterosis.md), [Outcross](https://genetixbay.com/knowledge/glossary/outcross.md), [Recessive](https://genetixbay.com/knowledge/glossary/recessive.md), [Genetic bottleneck](https://genetixbay.com/knowledge/glossary/genetic-bottleneck.md)

## Read more

- [Inbreeding depression and hybrid vigour](https://genetixbay.com/knowledge/breeding/inbreeding-depression-hybrid-vigour.md)
- [S1s and selfing: what happens when a plant pollinates itself](https://genetixbay.com/knowledge/breeding/s1-selfing.md)

## Sources

1. Charlesworth D, Willis JH. 2009. The genetics of inbreeding depression. Nature Reviews Genetics 10:783-796. https://doi.org/10.1038/nrg2664
2. Falconer DS, Mackay TFC. 1996. Introduction to Quantitative Genetics, 4th edition. Longman, Harlow. ISBN 978-0-582-24302-6