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Tasha genes offer canine clues

A BOXER named Tasha is set to help reveal why dogs have been man's trusted companions and hunting partners throughout recorded history.

Researchers have compiled the 12-year-old dog’s DNA recipe, bringing scientists a step closer to finding the genetic causes of diseases common to all mammals and identifying the differences between dog breeds.

When combined with an earlier genetic sequence of a poodle called Shadow, Tasha will help reveal why some dog breeds are perfect for petting and hugging, whilst others make fierce guardians and a few look like fluffy toys.

Researchers have been keen to unravel the DNA code of dogs because, through selective breeding, they have the highest degree of physical and behavioural differences seen within a single species, from 6lb Chihuahuas to 120lb Great Danes. As a result, some breeds are predisposed to conditions such as heart disease, cancer, deafness or blindness, and identifying genes responsible for diseases or traits should be much easier to do in dogs than man.

This week, an international consortium, including British teams from Oxford and Cambridge, and led by scientists from the Broad Institute of the Massachusetts Institute of Technology and Harvard, unveiled Tasha's DNA code in the journal Nature.

The sequence of 2.4 billion DNA "letters" records the genetic recipe, or genome, of the domestic dog (Canis familiaris), which consists of 19,300 genes - roughly the same number as that found in human beings. The team also sampled the genetic makeup of 10 dog breeds, as well as related species such as the grey wolf and the coyote, pinpointing 2.5 million differences in a single "letter" of genetic code, which serve as signposts to physical and behavioural traits, as well as diseases.

"Of the more than 5,500 mammals living today, dogs are arguably the most remarkable," said Prof Eric Lander, the report's senior author. "The incredible physical and behavioural diversity of dogs - from Chihuahuas to Great Danes - is encoded in their genomes. It can uniquely help us understand embryonic development, neurobiology, human disease and the basis of evolution."

By tracking evolution's genetic footprints through the dog, human and mouse genomes, the scientists found that humans share more ancestral DNA with dogs than with mice, confirming that dog genes can be used to understand human disease. They also found that selective breeding has shuffled large blocks of DNA code among dog breeds, which should make it easier to find the genes responsible for body size, behaviour and disease.

"The genetic contributions to many common diseases appear to be easier to uncover in dogs," said Dr Lindblad-Toh, the report's first author. "If so, it is a significant step forward in understanding the roots of genetic disease in both dogs and humans." The researchers compared Tasha's genetic code, which is around 99 per cent complete, with the 80 per cent complete code of the poodle Shadow.

It was the pet of Dr Craig Venter, who set up the Institute of Genomic Research (TIGR) in Rockville, Maryland, and led the private effort to record the human genetic code using his own DNA. In the journal Genome Research, TIGR researchers tracked short interspersed elements (SINEs) - stretches of DNA that occur randomly in genomes and can act as signposts for disease. The scientists found that the poodle and boxer differed in 10,562 locations.

Broadening the study to nine other dog breeds and five genomes of four types of wolves and a coyote, the scientists estimated that the dog population contains 20,000 SINE differences.