Scientists have decoded the complete genome of the domestic dog, a milestone announced yesterday that provides a biological roadmap for unraveling human diseases and probing the mysterious 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 the 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. Of the 10 most common diseases in dogs, eight are important to humans.

The completion of the dog genome offers the possibility that idiosyncratic dog breeds – often specifically bred for behavioral traits such as obedience, viciousness and 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 pure-bred 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.

Scientists refused to say much about Tasha, the pet of an unidentified family. Her stoic photo, however, now hangs prominently in the Mammalian Wall of Fame at the Broad Institute in Cambridge, Mass., where the sequencing work was completed.

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 to 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.

Among mammals, 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.