These are genetic disorders that affect a tiny proportion of people . 99.9% of the population falls into the standard XX/XY model.New Study Shows Why X And Y Chromosomes Alone Don't Determine a Baby's Sex
Scientists have discovered a new genetic regulator that plays a big role in determining whether a baby is born male or female, as well as whether or not their reproductive system is likely to develop differently after birth.www.sciencealert.com
From the article:
These disorders of sex development (DSD), while considered a controversial and unhelpful medical label for people who are simply 'intersex', are what the name suggests – conditions where the reproductive organs or genitals don't develop in a typical manner, leading to a mix of male and female characteristics.
The causes of these various intersex conditions aren't very well understood – something the new research should help with – and they affect around 1 in 4,500-5,500 people (though precise figures are hard to come by).