Scientists had already thought that DNA contained the answer for why dogs are such different sizes. Because each dog breed is always about the same size. For example, Golden Retrievers are usually 20-24 inches tall at the shoulders.
With that much consistency in size, DNA must be playing a role. They just didn't know what about the DNA caused the size variation.
To figure it out, the scientists used the Portuguese water dog. It comes in both small and large sizes making it an ideal dog to study. If there is a specific gene or bit of DNA responsible for differences between large and small dogs, it should be in the Portuguese water dog.
The scientists already knew that DNA in a region of chromosome 15 held the key to dog size. So they sequenced the DNA, or determined what each letter of DNA was, on that region of chromosome 15 in large and small Portuguese water dogs.
The sequencing revealed mutations or differences in the DNA between the large and small Portuguese water dogs. Some of the differences were very tiny. In fact, only a single letter of DNA was different in some places between the dogs.
These single letter differences are called single-nucleotide polymorphisms (SNPs pronounced "snips"). One common use for SNPs is determining who a baby's father is.
The scientists found 302 SNPs. That's a pretty small number considering they were looking at 15 million letters of DNA.
Then they looked at 116 of these SNPs in 463 Portuguese water dogs. The scientists used statistics to figure out how well each SNP correlated to the size of the dog's skeleton. The best correlation was found near the insulin-like growth factor (IGF) gene.
This was very encouraging because we already knew that IGF had something to do with body size. IGF interacts with almost every cell in the body. It controls cell growth and the way cells get energy.
Best of all, IGF influences the size of humans and mice. So maybe it plays the same role in dogs.
To figure this out, the scientists looked at IGF in 926 Portuguese water dogs. 96% of the dogs had one of two IGF types the "small" allele or the "large" allele. Small dogs usually had the small type. Large dogs usually had the large type.
But is IGF important in other dog breeds besides the Portuguese water dog? The scientists looked at IGF type in 23 small and 20 giant dog breeds. Only the small and large IGF alleles were significantly different between small and giant breeds.
Thus far, they'd only been dealing with small numbers of dogs. So to confirm that IGF helps determine dog breed size, the scientists looked at IGF type in 3241 dogs from 143 breeds.
Presto! A breed's IGF type predicted the average dog size of the breed. Within a breed, the percent of dogs that had the small IGF allele predicted how small that dog breed was.
So this one gene, IGF, is a major contributor to dog size. This finding is quite a surprise. Body size is very complicated. Scientists thought that many genes were probably involved.
And there are. IGF is a big determinant of overall breed size. Like the difference between a German Shepherd and a toy poodle.
But lots of genes are probably involved in making one German Shepherd a bit bigger or smaller than another one. This is how height works in people.
People are like one breed of dog - all about the same size. Numerous genes help determine height in people. Presumably like the other genes that contribute to differences in dog sizes within a breed.
Each of these genes only contributes a small bit to a person's (or dog's) height. Not like IGF which determines if a dog breed is short or tall.
So some people are tall because they have a lot of tall genes. Like basketball players. And others are shorter because they have fewer. Like jockeys.
Even considering jockeys and basketball players, people are really the same size. Compared to dogs anyway.