Caster Semenya’s easy victory in the 800m last week reignited rumours that she is actually a man, and prompted an official investigation that is unlikely to reach a firm conclusion for at least another couple of weeks. Determining gender is not always as simple as you might think.

Gender verification has improved since it was introduced in the Sixties amid concerns that men were masquerading as women to gain competitive advantage — such as the woman who broke world records at 400m and 800m, only to be recognised by her father as the son he thought he had lost in the Second World War. Or the 1966 women’s downhill world ski champion who went on to marry a woman and father a child.

In those days gender scrutiny was somewhat primitive and Semenya should count herself lucky that she doesn’t have to undergo the ignominy of parading naked in front of three gynaecologists, as female competitors in the 1966 European Athletics Championships had to do.

Instead, Semenya can expect a complex appraisal from a panel including a psychiatrist and an endocrinologist, which will reflect current understanding that there is more to gender than whether you have the right set of genitals. You don’t even need the right set of chromosomes.

Gender should be determined at the point of fertilisation. All ova carry an X chromosome, while only half of male sperm carry an X, the rest carrying a Y chromosome. If an X-carrying sperm reaches the ovum first, the resulting child will be XX and female. If a Y chromosome wins the race, the child will be XY and a boy. But it is not that simple.

The default gender for all developing babies is female and the Y chromosome alters this by increasing production of male hormones (androgens), which masculinise the child — the ovaries migrate through the abdominal wall to become the testes, the labia fuse to form the scrotum, the clitoris grows to become the penis and the child becomes a boy. But what if things don’t go to plan?

Several medical conditions can blur the boundaries between male and female, but perhaps the most graphic example is Androgen Insensitivity Syndrome (AIS), where the embryo carries male chromosomes (XY) but develops into a child that is outwardly female.

It all starts off well. The Y chromosome turns the default female ovaries into androgen-producing testes, but the rest of the embryonic tissues don’t respond to the hormones and continue along the female path. The result is often an outwardly female baby but with male chromosomes and internal testes instead of ovaries.

She — for that is how the child is perceived and brought up — will not start to stand out until puberty. Although she looks like a girl in all ways (including breasts), she is a male internally, so “she” has testes instead of ovaries, no womb and is missing the upper two thirds of the vagina. So she will have no periods, can never conceive and may have difficulty with sexual intercourse.

I have only knowingly seen one case of AIS. She was a married woman in her early twenties who presented at a fertility clinic because she was having no joy with her attempts to start a family. Imagine having to reveal the reason for her infertility — to both her and her husband — and you get some idea of the quandary that may face the experts assessing Caster Semenya.

But blurred gender is not just about genitals and chromosomes — it is about nurture, appearance and stereotypes such as dresses versus trousers. At one end of the spectrum you have naturally effete men and butch women. At the other are a host of rare and complicated medical conditions such as AIS; and in between there are common hormone disturbances such as polycystic ovarian syndrome (PCOS), which can masculinise some women (the bearded shot-putter).

It is not always easy to ascertain whether carrying a Y chromosome or having higher than normal levels of testosterone (as can happen in PCOS) confers any advantage in sport, which is why how an athlete is reared should be given at least as much weight in gender determination as his/her genetic profile.

Indeed, a meeting of the International Athletic Foundation in the early Nineties determined just that — namely, that female athletes should be allowed to compete if they were reared as girls / women, irrespective of their genetic sex or the appearance of their genitals.

Only those reared as boys /men — even if they turned out to be genetically female — should be automatically excluded.

With that in mind, I very much doubt that Semenya’s assessment will change her status as the current women’s 800 metres world champion. She may have masculine qualities but has obviously always thought herself a girl, and has been reared as such. Or, as a neighbour who grew up with her in South Africa put it bluntly on the Today programme: “We are quite sure she is a girl. No doubts — she wears panties.”

For an overview of the complexities involved in assessing gender, visit the FAQ section of the Intersex Society of North America website at www.isna.org