This article is a very good explanation of intersex athletes:
http://www.letsrun.com/news/2014/09/brief-history-intersex-athletes-sport/
(please read it)
In particular how there are two different categories:
[1] Androgen insensitivity syndrome (AIS)
[2] Five alpha reductase deficiency (5-ARD)
The article states:
"In each case, babies are born with the Y chromosome normally associated with maleness, but with mutations that often cause them to be assigned into the female gender. Despite outward appearances, however, people with either of these Differences of Sexual Development will not develop internal female organs due to the influence of the Y chromosome. Instead, their gonads will develop as testes, and they may or may not form other internal male organs.
AIS people have a mutation which prevents testosterone (T) from being absorbed by the body; hence there will be plenty of T in the blood, but virtually none in other cells. Those with AIS usually look like any other woman from the outside. Since there is no T uptake in the body, any athletic advantage they might have over other women is very small.
Those born with 5-ARD have a mutation which prevents the creation of dihydrotestosterone, or DHT, in the body. DHT is a powerful androgen which triggers the formation of male genitalia. Thus, 5-ARD babies are often assigned female gender. Unlike AIS people, however, those with 5-ARD are affected by the T in their system and become much more masculine at puberty. This can give them a very large athletic advantage over other women."
Therefore, those with AIS (eg Dutee Chand), have no real advantage. So that is what the Court for Arbitration of Sport (CAS) wanted the IAAF to prove. That the IAAF needs to show that people with AIS have any advantage. The answer is no they don't. They therefore should be allowed to compete, as is.
BUT!
Those with 5-ARD (eg Caster Semenya) have a significant advantage. To put things into perspective, normal testosterone range for men is 10 - 35 nmol/L (nanomole per liter), with an average in the low twenties, and for women it is 0.35 - 2.0 nmol/L, with an average of about 1.5.
The testosterone levels of athletes with 5-ARD was around 21 nmol/L. Ten times the level of the average female. This is the category that Caster Semenya is in! The CAS got it wrong. They grouped everyone in the same category. Ridiculous considering that there are two categories!