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Breeds: All
Description
DRD4 is the gene for the dopamine D4 receptor, one of the proteins that responds to dopamine in the brain. Dopamine signalling is involved in how an animal reacts to novelty, reward and stress, and variants of DRD4 have been studied in relation to exploratory and reactive behaviour across a range of species.
In the horse, researchers identified an A to G substitution in the repeat region of exon 3 of DRD4 and looked for a relationship between that variant and how horses behave. This test reports which version of that site your horse carries.
What the research found
The original study scored 136 Thoroughbreds on a set of temperament traits rated by the people who handled them daily. Two traits tracked with the DRD4 variant: horses that did not carry the A allele scored significantly higher for curiosity and significantly lower for vigilance than horses that did. The pattern held across subgroups separated by when the horses arrived at the farm.
A later, much smaller study of 21 stabled horses looked at behaviour when an expected feed was delayed. Horses without the A allele spent more of their time budget on frustration-related behaviour than horses carrying A.
Read together, those results do not line up as a simple calm-versus-excitable axis. The non-A horses were the more curious and less watchful group in the first study and the more frustration-reactive group in the second. What the variant appears to track is a difference in how horses engage with novelty and with thwarted expectation, not a ranking from good to difficult.
How to read your result
This is an association test, and it is worth being clear about what that means. Both studies compared group averages between horses that carry the A allele and horses that do not. They did not predict the behaviour of any individual horse, and the published analyses pooled A/A and A/G together, so the evidence does not distinguish one copy of A from two.
The sample sizes were modest and the horses were almost entirely Thoroughbreds. Temperament is shaped by many genes alongside handling, training, environment, health and age, and a single receptor variant accounts for only a small part of it. Treat this result as one piece of background information about your horse, not as a forecast of how it will behave or a basis for a breeding or purchase decision on its own.
Possible Results
| Genotype | Description |
|---|---|
| A/A | Horse carries two copies of the A allele. In the published work, A-carrying horses scored lower for curiosity and higher for vigilance as a group. |
| A/G | Horse carries one copy of the A allele and one copy of G. The published studies grouped A/G with A/A, so the same group findings apply. |
| G/G | Horse carries no copy of the A allele. As a group these horses scored higher for curiosity and lower for vigilance, and in a separate smaller study showed more frustration-related behaviour. |
Breeds Affected
All horse breeds can be tested. The published research was carried out almost entirely in Thoroughbreds, so how well the association holds in other breeds has not been established.
Reference
Association between equine temperament and polymorphisms in dopamine D4 receptor gene.
Mamm Genome. 2005 Jul;16(7):538-44. doi: 10.1007/s00335-005-0021-3.
PMID: 16151699
Animals (Basel). 2013 Jul 12;3(3):663-9. doi: 10.3390/ani3030663.
PMID: 26479526
Related Tests
The Equine Trait Panel runs DRD4 alongside Gait, LCORL, MSTN1 and MSTN2 in a single submission.