Study shows that contrary to long-held assumption, so-called inactive X chromosome in women isn’t so inactive after all.

The first comprehensive survey of gene activity in women’s X chromosomes has yielded unexpected findings that may help explain differences between the sexes.

Extensive variation discovered in gene activity in the sex chromosomes suggests that there are essentially two human genomes, male and female, says researcher Huntington Willard, director of the Duke University Institute for Genome Sciences & Policy (IGSP) in Durham, North Carolina.

Humans’ sexual identity is governed by sex chromosomes. Males have one X and one Y chromosome, while females have two Xs.

More than 45 years ago, researchers found that genes on one copy of the female’s X chromosome are switched off, which is known as X inactivation.

It has long been assumed that this inactivation results in complete silencing of the genes on the second X chromosome, leaving both males and females with one working X chromosome. Furthermore, scientists had assumed that the X chromosome in all women was identical.

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