Dog genome may also offer insight into his pal, man

Scientists have decoded the complete genome of the domestic dog, a milestone announced Wednesday that provides a biological roadmap for unraveling human diseases and probing the bond between man and his best friend.

Dozens of researchers worked for two years deciphering and analyzing the 19,300 genes belonging to a 12-year-old boxer named Tasha. What they found was an exceptional correlation between the DNA of Canis familiaris and Homo sapiens, according to a study published today in the journal Nature.

"Humans and dogs have essentially the same genes," said lead author Kerstin Lindblad-Toh, co-director of the genome sequencing and analysis program at the Broad Institute of MIT and Harvard University. "Every gene has a gene with the same function in the other genome."

That closeness is reflected in the numerous diseases shared by dogs and humans, including cancer, heart disease, blindness, epilepsy and diabetes.

The completion of the dog genome offers the possibility that idiosyncratic dog breeds — often specifically bred for behavioral traits such as obedience, viciousness or docility — may help illuminate the elusive genetic instructions that account for the infinite variability of human personalities.

Tasha, a stout female with a brown-and-white coat and drooping jowls, was selected from a group of 120 dogs screened by the National Human Genome Research Institute in Bethesda, Md. The purebred boxer was chosen because her genes showed the least amount of variation among the candidates. Only female dogs were considered because they have two X chromosomes, which researchers wanted to map in detail.

The researchers reported that the complete dog genome consists of 2.5 billion chemical letters — commonly known by the letters A, T, C and G — compared with about 3 billion for humans. In scouring Tasha's genome and comparing it to genetic data from 10 other breeds, they cataloged more than 2.5 million specific genetic differences that occur among dogs, producing wide ranges of sizes, shapes, temperaments and propensity for disease.

Dogs are a unique genetic specimen because of the intensive selective breeding that began only a few hundred years ago and created the roughly 400 breeds that exist today.

Before such breeding began in earnest, the chromosomes of dogs were as varied as in other animals. By mating close relatives to produce animals with specific traits — a kind of forced evolution — they erased much of the genetic diversity within each breed.

Since so many of the genes were the same, they were passed from generation to generation in unusually large chunks. Those large chunks, known as haplotypes, meant that genes controlling a variety of disparate traits were locked together. Thus, along with a pure black coat or an acute sense of smell, an entire breed of dogs might also inherit a particular disease.

Their unusual genetic architecture has made dogs particularly valuable for medical research. Scientists look for aberrant genes by comparing the DNA of subjects that either possess or lack a particular trait. When genes are contained in bigger haplotypes, they are easier to find.

The researchers found that haplotypes in dogs are 50 times bigger than those in humans, making it more efficient to hunt for disease-causing genes.

Dogs have already proved useful as proxies for humans in medical research. The gene that causes the sleeping disorder narcolepsy was first identified in Doberman pinschers and Labrador retrievers, and genetic defects associated with epilepsy came to light by studying pointers who suffered from the disease.

The dog's genetic code also contains glimpses into the unique relationship between humans and dogs.

Archaeological evidence shows that dog domestication was complete by 12,000 to 14,000 years ago. When humans crossed the Bering Strait land bridge to get from Asia to North America at the end of the last ice age, dogs came with them.

More than a decade ago, researchers tried to find the genes linked to canine behaviors. They didn't have much success. With the dog genome in hand, it's time to try again, said Elaine Ostrander, chief of cancer genetics at the National Human Genome Research Institute.

"Why do herding dogs herd, and why do pointers point?" she said. "The research community is really ready to tackle these challenges head-on."