The findings also highlight key differences between female and male genomes, according to the researchers.
For one thing, women are known to consist of a mosaic of two cell types that differ in which X chromosome is inactivated. The Y chromosome also endows males with at least several dozen expressed genes that females lack.
The incomplete nature of X inactivation, demonstrated in the new study, means that at least 15 percent of the X-linked genes, and their protein products, are present at characteristically higher, and often variable, levels in females compared to males.
Moreover, the findings show that a minimum of an additional 10 percent of genes are expressed at variable levels among females, while all males express a single copy of such genes.
"We now know that 25 percent of the X chromosome -- 200 to 300 genes ?can be uniquely expressed in one sex relative to the other," Willard said. "In essence, therefore, there is not one human genome, but two -- male and female.
"Such characteristic genomic differences should be recognized as a potential factor to explain sex-specific traits both in complex disease, as well as normal gender differences," he continued.
Notwithstanding the genomic and biological significance of these sex-specific differences, many questions remain, Willard said. For example, further study is required to determine if the pattern of X inactivation differs across tissues or over time and whether there are characteristic patterns of inactivation in maternally versus paternally inherited X chromosomes.