Can a DNA Test Predict How Big Your Dog Will Get?

Partly, yes. Body size is one of the most strongly genetic traits in dogs: a 2013 study found that variants near just six genes explain about half of the size differences between breeds. A DNA test can use genes like these to estimate a dog's adult height and weight. It's an estimate with a range rather than a promise, because many genes are involved, dogs carry different combinations of them, and food, health and growth play a part.

The short version

• Size is highly genetic, and a few genes have large effects.
• IGF1: one version of this gene is common to all small breeds and nearly absent from giant ones.
• Six genes, half the story: variants near six genes explain about half of the size differences between breeds.
• Breed differences aren't the same as one dog: dogs within a breed carry different combinations, so individual estimates are less certain.
• A DNA estimate is context, not a promise. Use it alongside your vet's growth checks.

The genes behind dog size

Dogs vary more in body size than any other land vertebrate, as the 2007 IGF1 study pointed out, and much of that comes down to a few genes:

• IGF1: a 2007 study found a single version of the IGF1 gene, which makes insulin-like growth factor 1, common to all small breeds and nearly absent from giant breeds. It's a major contributor to small size.
• Six genes together: a 2013 study of 500 dogs from 93 breeds looked at variants near GHR, HMGA2, SMAD2, STC2, IGF1 and IGF1R. The newer versions of all of them were linked to smaller body size, and smaller dogs tended to carry more of them. Wild canids carried the original versions.
• More still being found: a 2016 study of more than 4,200 dogs reported three additional regions of the genome that influence body size.

From breed differences to one dog's estimate

The 2013 study found that seven markers near those six genes explained roughly half of the variation in body size between breeds, and about two-thirds among breeds with standard weights under 41 kg. That's a lot for a handful of genes.

But those figures describe differences between breeds. The same study found that breeds aren't fixed for these variants: dogs within one breed carry different combinations. Mixed breeds carry even more variety. So predicting one particular dog's adult size from its DNA is less certain than explaining why a Chihuahua is smaller than a Labrador. That's why a genetic size estimate comes as a range to plan around, not a single number to count on.

How the PuppyGenes size estimate fits in

The PuppyGenes report's temperament and size predictions section includes an estimated adult height and weight based on your dog's own genetics: useful context, not a promise. In our public example reports, Juno is predicted at about 11.2 kg and about 41 cm tall, and Marley at about 12.8 kg and about 41 cm, and both reports note that the dog sizes up on the smaller side for its mix.

For a puppy, the estimate is most useful early on, before growth makes size obvious. Our guide to DNA testing a puppy covers when to swab, and our guide to reading a dog DNA report explains each section.

What DNA can't tell you about size

• How your puppy is actually growing. Growth standards built from more than 6 million young dogs, described in our guide to when dogs stop growing, track that. A 2020 study found dogs that became overweight had often grown faster than expected, so feeding matters.
• Your dog's ideal weight. A size estimate describes an expected frame, not a target to feed towards. Body condition is the better guide; our guide to checking whether a dog is overweight shows how.
• Giant breeds as well as others. The 2013 study's findings were strongest for breeds under about 41 kg, and its authors described the work as covering non-giant breeds, so the genetics of giant size are less fully mapped.

For a mixed-breed puppy, our guide to estimating adult size shows how to combine a DNA estimate with the parents' sizes and your puppy's growth line.