Polygenic Traits: Why Most Dog Traits Involve Many Genes

A polygenic trait is one shaped by many genes at once, each with a small effect, often alongside the environment. In dogs, body size and behaviour are polygenic, while furnishings, a long coat and merle are each mostly controlled by a single gene. That's why a single-gene trait like a beard is usually easier to predict from DNA than an adult weight.

The short version

• Many genes, small effects: a polygenic trait adds up the effects of many genes.
• Single-gene traits are different: furnishings (RSPO2), a long coat (FGF5) and merle (PMEL) mostly follow one gene.
• Size is polygenic, though a handful of genes have big effects.
• So is behaviour, and breed explains only a small share of the differences between individual dogs.
• DNA gives a range for polygenic traits, rather than a yes or no.

Single-gene vs polygenic traits

Single-gene traits follow the simple odds in our guide to dominant vs recessive genes. Furnishings are a clear example: a study of more than 1,000 dogs from 80 breeds found that 297 of 298 furnished dogs carried the same RSPO2 insertion, as our furnishings guide explains.

Size: a few big genes and many small ones

Body size shows how a polygenic trait can still have a few genes with large effects. A 2007 study found that a single IGF1 haplotype was common to all small breeds and nearly absent from giant breeds. A 2013 study found that seven markers near six genes explained 46% to 52.5% of the variation in body size between breeds, and that most breeds carried a mix of genotypes at those markers rather than one fixed set.

That leaves about half the difference between breeds to other genes and to the environment, which is why a DNA size estimate comes with a range. Our guide to whether DNA can predict adult size explains how that estimate works.

Behaviour: many genes, and breed explains little

Behaviour is polygenic too. A 2019 study of more than 14,000 dogs from 101 breeds found 131 SNPs associated with breed differences in behaviour, in genes highly active in the brain. A 2022 study of more than 18,000 dogs found that most behavioural traits are heritable, but breed explained only about 9% of the behavioural variation between individual dogs. Its authors proposed that typical breed behaviours come from polygenic adaptation that predates modern breeds.

Our guides to heritability and what a dog's breed means for daily care explain how to read findings like these.

What it means for DNA results

For a single-gene trait, a DNA report can say which versions of the gene a dog carries and what they usually mean. For a polygenic trait, it can only estimate. The PuppyGenes temperament and size predictions section gives breed-informed tendencies and an estimated adult height and weight: useful context, not a promise. In Juno's example report, the predicted adult size is about 11.2 kg and about 41 cm tall.