Two X chromosomes (XX) makes women female. Men have an X and a Y, the latter of which is responsible for all the things that make them male. What we teach is that one of the X’s in the women’s XX’s dominates the other so that women’s physical characteristics (phenotype) is only determined by the dominant X. Recent research shows that’s not necessarily the case.
“Our study shows that the inactive X in women is not as silent as we thought,” said Laura Carrel, Ph.D., assistant professor of biochemistry and molecular biology, Penn State College of Medicine, Penn State Milton S. Hershey Medical Center. “The effects of these genes from the inactive X chromosome could explain some of the differences between men and women that aren’t attributable to sex hormones.” – From Penn State University, edited by Science Daily.
It’s a fascinating finding, but I don’t think I’ll go into it with the students except, in the very unlikely event, that a direct question comes up. At this point, simpler is better.