Sex is supposed to be simple, at least at the molecular level. X + X = female and X + Y = male.
But as science looks more closely, it becomes increasingly clear that a pair of chromosomes is not always sufficient to distinguish girl/boy—either from the standpoint of sex (biological traits) or of gender (social identity).
In the cultural realm, this shift in perspective has received a wide embrace, with “nonbinary” definitions of gender—transfeminine, genderqueer—having already entered the vernacular. Less visible are the changes taking place in the biological sciences.
The emerging picture of “girlness” or “boyness” reveals the involvement of complex gene networks, extending far beyond the moment six weeks after gestation when the gonads begin to form.
To different extents, many of us are biological hybrids on a male-female continuum. New evidence suggests that the brain consists of a “mosaic” of cell types.
For societies in which gender inequality persists, the new science of sex and gender may help shape public perception and governmental policy, making them better acknowledge this reality.
To explain to students what a continuum is, I use the example of the light spectrum: "There's not only black and white, but many colours in between." Using pictures also helps to explain several of the words, too, e.g., pictures of the light spectrum, the yin/yang symbol, a mosaic, a chromosome. When it comes to the 3rd question, you can talk about it in both a global and a Japanese context. Regarding the 4th question, some families (notably in Sweden) are choosing to raise their children without gender. If this topic interests your students, you can recommend that they watch this video for homework: Raised Without Gender