BIOLOGISTS FINISH MAPPING FEATURES OF X CHROMOSOME.

Byline: Nicholas Wade The New York Times

Reaching a significant goal in the project to explore the mysteries of human genes, biologists have completed a high-resolution map of the X chromosome, one of the pair that determines whether a baby is a boy or a girl.

The map consists of a set of identifiable milestones at frequent intervals along the chromosome, which is a giant molecule of DNA some 160 million chemical units in length.

Although rough maps of the X and other chromosomes have been made before, this is the first time that any chromosome has been mapped to the level of detail set by the Human Genome Project, a $3 billion effort to describe the human genetic blueprint completely, said Eric Green of the National Human Genome Research Institute.

The map, which took 10 years to complete, is the work of 25 biologists at the Washington University School of Medicine in St. Louis. The team's work, directed by David Schlessinger, was reported Friday in the journal Genome Research.

The X chromosome may to many people connote femaleness, since women have a pair of X chromosomes and men have an X and a Y.

But it is of intense interest to geneticists because of a reason that has to do with males: the fact that many conditions caused by defective genes on the X chromosome, like hemophilia and color blindness, turn up only in men. The reason is that women can often compensate for a defective X chromosome gene if the counterpart gene on their second X chromosome is in working order.

David Nelson, an X-chromosome expert at the Baylor College of Medicine, described the map produced by Schlessinger's team as a ``tour de force.''

Huntington Willard, a geneticist at Case Western Reserve University, said the map ``will greatly speed up searches for X-linked disease genes and efforts to understand chromosome biology.''

The map will enable other laboratories to sequence, or chemically identify, the DNA units in the regions between the map's milestones, which are themselves short sequences of DNA about 75,000 units apart. By filling in all the gaps between the milestones, the full DNA sequence of the X chromosome can be determined, giving a complete description of the human genetic blueprint in at least one of the 24 chromosomes that make up the human genome.

The map is also a gift to researchers studying X-linked diseases. Without waiting for the full DNA sequence, they can now locate a gene's likely position on the X chromosome more rapidly by identifying the milestones that are most closely inherited along with the gene of interest.