Twilight, a grey thoroughbred mare raised at Cornell University, was the DNA donor for the genome project. (Science/AAAS) An old grey mare named Twilight has provided an international team of researchers with the entire genetic code of a domestic horse.

The scientists say the genome sequencing has revealed more information about its evolution and domestication, and shows some similarities with other known genomes, such as those of the cow and even the human.

Twilight, an adult female thoroughbred horse from Cornell University, was the DNA donor for the genome project, published this week in the journal Science.

The DNA was analyzed and the horse genome was found to be about 2.7 billion nucleotides long, slightly larger than the genome of the domestic dog, and smaller than the genomes of both humans and cows.

Having the entire genome of the horse could lead to new breeding techniques, as well as information on diseases in both horses and humans. More than 90 hereditary diseases in horses show similarities to those in humans.

"Horses and humans suffer from similar illnesses, so identifying the genetic culprits in horses promises to deepen our knowledge of disease in both organisms," said senior author Kerstin Lindblad-Toh of the Broad Institute of the Massachusetts Institute of Technology and Harvard, and Uppsala University in Sweden.

The genome has already allowed scientists to develop tests for some equine genetic disorders.

One unusual aspect of the horse genome was a structure, called a centromere, that appears to have developed on chromosome 11.

The centromere is the central region of a chromosome where the arms constrict and come together to form an "X" shape. Centromeres ensure that when a cell divides, each new cell gets a copy of the chromosome.

The new centromere, which appears to be frozen at an early stage, is fully functional and appears in no other species, suggesting that it is a relatively new evolution.

"We don't know a lot about centromeres, particularly because they have proven so difficult to analyze by DNA sequencing," said co-author Claire Wade of the University of Sydney in Australia. "This result helps address some important questions about how centromeres evolve."