on February 24th, 2015 1 Comment
Imagine being a forty-six-year-old woman pregnant with her third child, whose amniocentesis follow-up shows that half her cells carry male chromosomes. Or a seventy-year-old father of three who learns during a hernia repair that he has a uterus. A recent news feature in Nature mentioned these cases as it elaborated on the spectrum of sex biology. People can be sexed in a non-straightforward way and not even be aware of it; in fact, most probably aren’t. As many as 1 person in 100 has some form of “DSD,” a difference/disorder of sex development.
The simple scenario many of us learned in school is that two X chromosomes make someone female, and an X and a Y chromosome make someone male. These are simplistic ways of thinking about what is scientifically very complex. Anatomy, hormones, cells, and chromosomes (not to mention personal identity convictions) are actually not usually aligned with one binary classification.
The Nature feature collects research that has changed the way biologists understand sex. New technologies in DNA sequencing and cell biology are revealing that chromosomal sex is a process, not an assignation.
As quoted in the article, Eric Vilain, MD, PhD, director of the Center for Gender-Based Biology at UCLA, explains that sex determination is a contest between two opposing networks of gene activity. Changes in the activity or amounts of molecules in the networks can sway the embryo towards or away from the sex seemingly spelled out by the chromosomes. “It has been, in a sense, a philosophical change in our way of looking at sex; that it’s a balance.”
What’s more, studies in mice are showing that the balance of sex manifestation can be shifted even after birth; in fact, it is something actively maintained during the mouse’s whole life.
According to the Nature feature, true intersex disorders, such as those from divergent genes or the inability of cellular receptors to respond to hormones, yield conflicting chromosomal and anatomical sex. But these are rare, about 1 in 4,500. For the 1/100 figure, they used a more inclusive definition of DSDs. More than 25 genes that affect sex development have now been identified, and they have a wide range of variations that affect people in subtle ways. Many differences aren’t even noticed until incidental medical encounters, such as in the opening scenarios (the first was probably caused by twin embryos fusing in the woman’s mother’s womb; the second by a hormonal disorder).