Children's Health

Genetic Scientists Put Down's Syndrome on the Map

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"It's not just a simple string of 225 genes. It's really a mess, a hornet's nest -- a hodgepodge of duplications, altered sequences, and arrangements that determine the health and welfare of our species."

Reeves tells WebMD that this new information, combined with what is now known about chromosome 22, hints that humans may have only half of the 70,000 or 100,000 genes that scientists had speculated.

"To have the [chromosome] 21 consortium announce that there were only 225 genes is quite a surprise," Reeves adds. "In addition, they went another step and combined their information with that from chromosome 22, and they projected there may be only 40,000 human genes in all."

The researchers found that chromosome 21 contains far fewer genes than the 545 in chromosome 22, the second-smallest chromosome.

The map of chromosome 21 is 99.7% complete; technical limitations prevented a complete mapping, says Patterson.

Rudolph Tanzi, a professor of neurology at Harvard Medical School, said the chromosome map could eventually shed light on why people with Down's syndrome have very low rates of breast, lung, and gastrointestinal cancers. It could be that an extra copy of chromosome 21 has tumor-suppressing qualities.

"One can argue that an extra dose of a gene will usually be a bad thing, but once in a while it can be a good thing. It can be protective," Tanzi said.

The Human Genome Project expects to have a rough draft of the entire human genetic blueprint done this summer. The public project, which is expected to finish its work by 2003, is competing against a private company, Celera Genomics Corp., of Rockville, Md., which hopes to sell the information to pharmaceutical companies and others.