What Dogs Tell Us: The ABC's of DNA

HOW TO BUILD A DOG

This National Geographic article, with an internet copy attached, looks into the genetic "recipes" that create the 350 or so dog breeds out there. The large array of body sizes, ear shapes, nose lengths, and barking habits make dogs the most diverse animal on the planet. If humans had as much diversity as dogs, the smallest people would be 2 feet tall fully grown (like a chihuahua) and some would be 31 feet tall (like a Saint Bernard)! 

The most impressive part of all this diversity is how quickly it came about. There are about 350-400 dog breeds, and most have only been around for a couple of hundred of years. Somehow the variation occurred, occurred fast, and stuck. When it comes to dogs, breeders have pretty much put natural selection into fast-forward by first, combining desired traits from disparate dogs and then, breeding the offspring with the largest hint of that desired trait. 

The Dachshund

The demand for a better badger hunting dog led to the artificial evolution of the dachshund, which literally means "badger dog" in German. For a dog better at cornering badgers, breeders used a combination of hounds. The obvious choice was the basset hound, with its stubby legs and long body, and they bred it with a mixture of terriers for speed to chase its prey into a burrow. After a couple of generations, they got the dachshund. Its pliable skin serves as a defense mechanism, it can get bit without significant damage or harm. And the dachshund's long, sturdy tail makes it easier for hunters to retrieve the dog, with a badger in it's mouth, from a badgers burrow. 

The variation in the dachshund is only the beginning of the variation found throughout the entire spectrum of dogs. It was once believed, and understandably so, that the morphologic diversity of dogs was mirrored in the canine genome. What researchers have found found though is that it is actually quite the opposite. It turns out that the vast mosaic of dog shapes, colors, and sizes are determined largely by a changes/mutations in just a handful of genes. There are only about 50 genetic switches involved in the breed's distinct appearances. It turns out that the difference between the dachshund's small body stature and a Rottweiler's massive one lies in just a single gene. Even the dachshund's stumpy legs, a mutation known as disproportionate dwarfism or chondrodysplasia, also lies in one gene. Some genes have been specifically identified. Like the gene that determines if a dog has floppy ears, like a basset hound, or erect ears, like a great dane, lies on canine chromosome and is called CFA10.

The wrinkles that give shar pei's their distinct look are in the gene region labeled HAS2. 

The Rhodesian ridgeback has a unique stretch of fur on it's back that grows in the opposite direction, hence the name ridgeback. This unique trait lies in a mutational change of gene CFA18. 

Genes that govern a dog's coat

This was the diagram I found most interesting in this article. It illustrates, very clearly, the 3 genes that play a role in a dog's coat or fur. The three genes are KRT71 (the blue oval), FGF5 (the green oval), and RSPO2 (the red oval). The normal interaction of these three genes result in a short, smooth coat like that of a labrador retriever, but when one or more of these genes is mutated, there is a variation in coats. A mutation in the RSPO2 gene results in a wire coat and/or furnishings (extended beards and eyebrows) like the Australian terrier. A single mutation in the FGF5 gene results in a long coat, such as a Golden retriever, and a mutation in the KRT71 gene results in a curly coat. There can be multiple mutations though. The bearded collie has a mutation in the FGF5 and RSPO2, giving it a long coat with furnishings. The Airedale has two mutations leading to a curly and wiry coat with furnishings, or the Irish water spaniel who has a long and curly coat. And the Bichon frise has a mutation in all three genes giving it a curly, long coat with furnishings. Even with this simple example, you can start to see how much variation is possible with just a few genes. It is not hard to see that if this concept is expanded to more traits, each with a couple of genes, it is very easy to see how all the variation has been made possible.