Now one can argue that chromosomes aren't definitive; that both XX males and XY females exist. But the thing is... the argument that chromosomes can't be changed is just plain wrong. Chromosomes can be changed, and are changed, as the result of various medical therapies.
From Transplanted human bone marrow cells generate new brain cells by Crain BJ, Tran SD, Mezey E. in J Neurol Sci. 2005 Jun 15;233(1-2):121-3 :
Multiple studies have reported that adult cells of bone marrow origin can differentiate into muscle, skin, liver, lung, epithelial cells, and neurons. To determine whether such cells might produce neurons and other cells in the human brain, we examined paraffin sections from female patients who had received bone marrow transplants from male donors. Y-chromosomes were labeled using autoradiography and fluorescent in situ hybridization. Neurons and astrocytes were identified histologically and immunohistochemically in neocortex, hippocampus, striatum, and cerebellum. However, most labeled cells in both gray and white matter appeared to be glia. Others have suggested that such Y-labeling represents fusion between host and donor cells, rather than true transdifferentiation. The possibilities of fusion and microchimerism were therefore examined using buccal epithelial cells as a model system. The female patients in this study had received either bone marrow or stem cell (CD34+ enriched) transplants from their brothers. Double labeling for X- and Y-chromosomes showed that Y-labeled buccal cells could not be explained by fusion. Genotyping studies of one patient, her brother, and her son ruled out the possibility of microchimerism. Whether, and under what circumstances, some form of bone marrow transplantation might provide adequate number of cells capable of replacing lost brain cells or enhancing their function will require additional studies.And then there's this : Bone marrow-derived cells from male donors can compose endometrial glands in female transplant recipients by Ikoma et al in Am J Obstet Gynecol. 2009 Dec;201(6):608.e1-8 :
RESULTS: All recipients had donor-derived Y chromosome-positive endometrial cells, accounting for 0.6-8.4% of glandular epithelial cells and 8.2-9.8% of stromal cells. Most of the endometrial glands were chimeric, consisting of both donor-derived and recipient cells.M.Italiano even gives several methods where something like this can be accomplished deliberately, rather than observed as a surprising side-effect.
CONCLUSION: Donor-derived cells are capable of composing endometrium in recipients, even those of the opposite sex.
Our lives are like theatres, our bodies like long-running plays. Every day, each performance is just slightly different. The cast will change over time. Cells are replaced. We're each of us, just slightly different people when we wake up in the morning than the person who went to bed. If not, growth would be impossible, learning and memory likewise.
Many people, I think, would consider this idea threatening to their identity. They see themselves as static portraits, rather than performance art.