Dear Dr Milrod:

I’ve been thinking that women are basically “hard-wired” to consider as many angles as possible when it comes to “mating” whereas men have basically one (to a point (g)). What’s your take on it?

Mating For Fun

Dear Mating For Fun:

Indeed, you’re on to something. In 2005, the journal Nature, published some startling studies on the second X chromosome of women – the one previously thought “silent.” But this chromosome may account for the ‘unpredictable’ variation in women..and the more solipsistic expression of genes in men.
For every man who thinks women are complex, there’s new evidence they’re correct; at least when it comes to their genes.

Men have one X chromosome and one Y chromosome, the latter being responsible for the characteristics that make men male, including the male sexual organs and the ability to produce sperm. In contrast, women have two copies of the X chromosome. But, because the X chromosome carries a bigger instruction manual than the Y chromosome, biology’s solution is to largely inactivate one X chromosome in females, giving one functional copy of the X in both men and women. But the study found that the “inactive” X in women wasn’t so passive after all. 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.

Depending on the gene, having two active copies can matter very little, or very much. When genes on the inactive X escape inactivation and are expressed, that can create a stronger overall concentration of particular genes. The study determined which genes were escaping inactivation and where they were located on the inactive X chromosome. They found that most of the wayward genes were grouped together.

A two-lab system was developed to investigate the inactive X chromosome. Using primary skin cells, gene expression was compared between the X chromosome and the inactive chromosome for 94 genes spanning the X chromosome in 40 human samples. It was found that only 65 percent of the genes were inactive in all samples. Twenty percent were inactivated in some samples and not in others, and 15 percent escaped inactivation in all samples. In addition, many of those from the inactive X that were expressed were only partially expressed.

The second laboratory system used other cell lines to compare inactive to active X chromosomes and recorded genes expressed from inactive X chromosomes. Six hundred twenty-four genes on the chromosome were tested with this system and also showed that 16 percent of genes on the inactive X escaped inactivation, confirming the results in the first laboratory model.

Although this data was collected from cell culture experiments, it neatly explains that there is more variability among females than scientists thought.

The data also suggests that the female genome now differs from the male genome in at least four ways. First, previous studies had shown that the Y chromosome gives males several genes that are absent in the female. Second, this study shows the fact that some genes on the inactive X are expressed means that about 15 percent of the genes are expressed at higher levels in females than in males. Third, this study also shows an additional 10 percent of genes on the inactive X show variable expression levels in females, whereas men have only a single copy of these genes. And finally, scientists had already known that random nature of X-inactivation shows that females, but not males, are mosaics of two cell populations with respect to X-linked gene expression.

It can be concluded that these differences should be recognized as potential factors for explaining normal differences between the sexes but also gender differences in how certain diseases are manifested, progress and respond to treatment. So what does this mean in real life – well, it could mean exactly what you’re suggesting. Woman is fickle like a feather in the wind!

Christine Milrod, PhD