"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).
Autism appears in four times as many men as women, so scientists have long suspected that the
X chromosome contains relevant genes.
In a female embryo, the choice of which
X chromosome to inactivate--either that from the mother or that from the father--is random in each cell.
The reason female mammals bother inactivating an
X chromosome is self-preservation.
When the researchers analyzed the sperm in the X collection tube, they found that 85 percent had the
X chromosome, as desired.
The genes have counterparts on the
X chromosome, which means that two working copies of each gene may be required for good health.
Women typically have two
X chromosomes, one inherited from each parent; men receive an
X chromosome from their mother and a Y chromosome from their father.
The genetic activity of a single
X chromosome is sufficient for a human cell, but women must somehow deal with a double dose.
Scientists believe this male chromosome once had thousands of genes in common with the
X chromosome, the other mammalian sex chromosome.
In a reprise of the most controversial study of 1993, researchers surveying the genetic landscape of the
X chromosome have discovered further evidence that it contains a gene or genes that may steer some men to homosexuality.
Abnormal genes that are located on
X chromosomes result in X-linked disorders, like haemophilia.
Chromosomes of Group XIII could be identified from Group IX and
X chromosomes by the large satellite on the short arm.