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. Last Updated: 07/27/2016

Project Decodes 2nd Chromosome

The international Human Genome Project reported Monday that it has finished decoding the human chromosome responsible for Down syndrome, the most common genetic cause of mental retardation.

The "essentially finished" genetic code of chromosome 21, published in the journal Nature, is viewed as an important step by genetics researchers because of the new insights it offers into a disorder that strikes every racial and ethnic group in the world.

In the United States alone, more than 350,000 people have Down syndrome. One child in every 700 to 1,000 births is born with the condition. In addition to retardation, these individuals often suffer heart defects, blockages in the digestive tract and lung infections. They also have a high incidence of leukemia.

Most people with Down syndrome have an extra copy of chromosome 21 in every cell in their bodies, and identifying all the genes carried on that chromosome will help scientists understand the disorder and its many variations.

"We will not have a gene therapy tomorrow as the result of finding the sequence today," said Roger Reeves, a geneticist at Johns Hopkins University School of Medicine. "But it will allow the research community to greatly focus on a small set of genes as the primary cause of what goes wrong in Down syndrome."

This is the second chromosome that the publicly funded Human Genome Project has deciphered so far, and it is the smallest of all the human chromosomes.

The researchers found that chromosome 21 contains far fewer genes than the 545 in chromosome 22, the second-smallest chromosome. The relative sparseness of genes in chromosome 21 may mean the total number of genes in human DNA is under 40,000 - not the 100,000 or more that had previously been thought - said Andre Rosenthal, a professor at the Institute for Molecular Biotechnology in Jena, Germany.

That means "we are not so different from drosophila" - the fruit fly - "or yeast," Rosenthal said.

The map of chromosome 21 is 99.7 percent complete; technical limitations prevented a complete mapping, said David Patterson, an American scientist involved in the project

Deciphering or sequencing a chromosome means determining the order of millions of chemical building blocks in DNA, the molecule that carries the genetic code.

The information will help scientists discover the number, placement and function of genes - long stretches of DNA that most often contain the directions for manufacturing one or more protein needed for growth and maintenance of health.

Scientists believe that cracking the genetic code will lead to new treatments and methods of diagnosis for many human ills, including cancer and heart disease, and perhaps the effects of aging.

The 16 centers in the international public consortium announced separately that they have all but completed their "working draft" of the human genetic code and are beginning to fill in the missing pieces to produce a more finished version of all the human chromosomes by 2003.

The first draft, expected to be available on a public web site this June, will include 90 percent of the code from a reference set of 24 chromosomes - numbers 1 through 22 plus the X and Y chromosomes that determine gender - drawn from 10 anonymous donors.

The announcements come amid growing expectations that a biotech company, Celera Genomics, will announce its draft of the genome - the entire genetic code of an anonymous individual - over the next several days.