What Determines Your Dog's Size?

Have you ever wondered what makes small dogs small and what makes big dogs big? Well a group of scientists wondered how dogs could have the biggest range of size of any mammal in existence today and their findings are published in this month’s issue of Science.

To find the answer, the scientists ran DNA profiles and took measurements of a group of Portuguese Water Dogs. Why Portuguese Water Dogs? Because the breed has a distinct size range and there are small, medium and large sized Portuguese Water Dogs.

The researchers found a regulatory sequence that appears next to a gene that regulates growth in dogs. In small dogs, mutations in the regulatory sequence suppress the growth gene which causes small dogs to remain small. In large dogs, the mutations are not present and the growth gene is not restricted.

To prove their findings, the scientists expanded their subjects to other breeds of dogs and they found the same mutations of the regulatory sequence in other small breeds.

The mutation in the regulatory gene is what ensures that Chihuahuas don’t grow up to be the size of a Mastiff. Considering that all dogs descended from wolves and wolves do not have the same genetic mutation of the regulatory sequence that small dogs posses, it is very interesting to note the differences between dog breeds. At some point, after dogs became domesticated, the genetic instruction that gives small dogs their size must have been introduced. At this point, there is no concrete research that tells us when this genetic instruction was introduced. If the gene was not introduced to small dogs, the only other possibility is that small dogs are descendants of a smaller wolf.