We do not yet know what gene on the
X chromosome is causing this effect on behaviour," Bonthuis said, but only a small percent of genes are expressed on both
X chromosomes (some genes are automatically turned off in one X when two
X chromosomes are present).
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.
Unchecked, Xist RNA sets off a cascade of molecular events that compacts the
X chromosome into inert material.
In one recent study, for example, biologists found that many more
X chromosome genes than expected escape inactivation.
Marked social difficulties and academic failure occurred far more often in the group with a maternally inherited
X chromosome, the researchers report in the June 12 Nature.
Furthermore, the RNA strands made by Xist cover the inactivated
X chromosome "like a sheath," says Rudolf Jaenisch of the Whitehead Institute for Biomedical Research in Cambridge, Mass.
Two-thirds of the 32 pairs of homosexual brothers included in the final analysis share the same markers for one portion of their
X chromosome and thus inherited the same span, Hamer and his colleagues at the University of Colorado in Boulder and the Whitehead Institute for Biomedical Research in Cambridge, Mass.
Willard and his colleagues compared DNA taken from inactive human
X chromosomes with that taken from active human
X chromosomes.
In theory, because his mother's
X chromosomes carried no hemophilia genes, the boy had no chance of being born a hemophiliac.
Another marker on the
X chromosome has also been associated with manic depression (SN: 6/13/87, p.
They also found what appears to be a related gene on the
X chromosome.
Julien Mendlewicz of the Free University of Brussels,Belgium, and his colleagues now report that there is another manic depression marker in the same area of the
X chromosome.
