Women now have another way to differentiate themselves from men.

The chromosomal difference between the sexes is larger than originally thought, according to a study released last week by a team of international researchers that was co-authored by a Penn State professor.

Chromosomes are the small packets of information in DNA that determine how an organism will develop. The X and Y chromosomes determine gender.

"The X chromosome is pretty big, consisting of almost 1,100 genes," while the Y chromosome is much smaller, having about one third the number of genes, said Laura Carrel, assistant professor of biochemistry and molecular biology at the College of Medicine.

The combination of an X chromosome and a Y chromosome will make an organism male, while two X's will make it female.

Normally, one of the X chromosomes in females is inactivated or "turned off" so that women and men will have similar doses of X-associated genes, which are thought to be important in development, Carrel said.

This means that females could have two times the amount of genetic information if one of their X chromosomes were not turned off before development, Carrel said.

"It is known that some genes escape this inactivation," she said.

In her most recent study, Carrel and Huntington Willard, of the Institute for Genome Sciences & Policy at Duke University, were able to determine which genes were escaping the inactivation.

There are three types of genes that remain active, said Sharon Shriver, assistant professor of biology. The pseudoautosomal gene is needed for pairing in cell division, the X-inactivation gene keeps the chromosome inactive, and then there are a handful of other genes that are not actively used but can still be activated.

"There are 'leaky' genes on the inactive chromosome," Shiver said, "genes where researchers are able to see a little expression."

In each of the 40 human female subjects in the study, 94 genes on the X chromosome were studied for expression from the inactive chromosome, Carrel said. Of the genes studied, 15 percent were the same in all of the subjects, while 10 percent were expressed in some, but not all, of the subjects and at variable levels.

"That handful of genes is much larger than researchers originally thought," Carrel said.

This inactive expression means that some of the genes in females will be expressed at a higher level than in males, she added.

Carrel said that group of genes that were expressed variably is what causes differences between females.

One unique finding in the study was that the genes that remain expressed on the inactive chromosome cluster together, Carrel said.

"They tend to be in areas that are evolutionarily more recent," she added.

In some cases, the extra expression could lead to no noticeable changes, Carrel said. However, in other areas, the extra protein could have a significant impact on the genetic development.

These genes should be recognized as potential factors in how certain diseases become manifest in females, she said.

With the recent completion of the X chromosome sequence, researchers may get a better understanding of this phenomenon, Shriver said.