Dogs vary in size due to variations in one gene

From the smallest Chihuahua to the largest Great Dane, dogs dramatically vary in size, much more than most other creatures and an international team of scientists reports that it has found the reason is down to a scrap of DNA.

Ranging from toy to giant, dogs have the biggest size variation of any mammal and the team of US and British experts believes it is due to variations in one gene.

After analysing DNA samples from 3,200 dogs, they pinpointed IGF-1, a gene that makes a hormone called Insulin-like Growth Factor 1. Variants of the gene were very strongly associated with the different sizes of dogs and the scientists are now trying to determine the precise mutations occurring within the gene.

The scientists began by looking at Portuguese water dogs, which range from 25 pounds to 75 pounds. Small and large Portuguese water dogs had one region of their DNA that differed. It was here that the IGF-1 gene, which is known to influence body size in humans, was found.

Hormone factor

The experts then analysed the same region in hundreds of dogs from 14 small breeds. The IGF1 gene's 2019s hormone helps humans and other mammals grow from birth to adolescence.

Medium and large dogs have the IGF1 gene but lack the newly discovered piece of DNA. In small dogs, however, this scrap of DNA next to the IGF1 gene suppresses the gene's 2019s activity, keeping small dogs from growing larger.

The study is relevant to human medicine because a vast majority of the genes in the human genome can also be found in the dog genome. The team, which reported its findings in the journal Science, was led by geneticists Elaine Ostrander and Nathan Sutter. Sutter told The Daily Telegraph: "There is strong support that IGF1 does promote growth and help determine body size in people. One example is a boy who was born without any IGF1 gene function; he was born extremely small and grew very slowly."

The research could help scientists looking for the genetic causes of human diseases such as arthritis, diabetes and cancer, said the researchers.