Why a person feels male when they are biologically female, or the other way around remains uncertain. Embryos start to become male or female at about six to eight weeks.

At that time, those with an active gene called SRY starts to produce the male sex hormon, testosterone.

Without the hormon, embryos remain female. With testosterone, masculinisation begins. It is the fork in the road that shapes a person’s anatomy and physiology, and potentially their behaviour.

By the time they reach puberty, 75% of children who have questioned their gender will identify with the one they were assigned in the womb.

A person’s sexual identity can be thought of as a product of four related factors: their biological sex; their sexual orientation; the gender they feel; and the gender that dominates the way they behave.

Sexual orientation is who someone is sexually attracted to. Gender identity is who someone is.

The various elements of what we consider male and female do not always line up neatly, with all the XX’s – complete with ovaries, vagina, estrogen, female gender identity, and feminine behavior- on one side and all the XY’s – testes, penis, testosterone, male gender identity, and masculine behavior – on the other.

It is possible to be XX and mostly male in terms of anatomy, physiology, and psychology, just as it is possible to be XY and mostly female.

Gender is an amalgamation of several elements: chromosomes (those X’s and Y’s), anatomy (internal sex organs and external genitals), hormones (relative levels of testosterone and estrogen), psychology (self-defined gender identity), and culture (socially defined gender behaviors).

And sometimes people who are born with the chromosomes and genitals of one sex realize that they are transgender, meaning they have an internal gender identity that aligns with the opposite sex – or even, occasionally, with neither gender or with no gender at all.