Why is it okay to discriminate on the basis of intelligence?

No one can really define what intelligence means, for a start. There is now lots of evidence that emotions and intelligence are linked, for example, and it is pretty well established that the way we manage our emotions is a product of upbringing and attachment. There are many studies showing that the ability to manage emotions and to think are linked and also linked to social factors.

Apparently, discrimination on the basis of physical features like skin colour, height and attractiveness is frowned upon but it is okay to discriminate on the basis of intelligence. Why is that, given that physical features and intelligence are both genetically predetermined?

I don't think it is ok to discriminate against people based on intelligence, except of course when it comes to aptitude. Intelligence is of course determined by an interplay between nature and nurture, and as such is not the result of free decision by the individual. Some people get angry at people for being stupid, or even actively dislike them, when of course who would want to be stupid? Only a stupid person, after all, would not see the value of intelligence.

Not all discrimination is harmful discrimation though - it's perfectly acceptable to discriminate against black people when casting for a white person's role for a historical TV production. In the same way it is fine to discriminate against a non-intelligent person for a job role which requires a certain level of intelligence. Intelligence affects candidacy, and so therefore is appropriate to be considered as a criterion to base a judgement of selection.

Just as it would be perfectly alright to discriminate against a white person for a black and asian casted role because of cultural and historical factors

Oh wait this did not happen:
Dragonball Evolution
Avatar the Last Air bender
The black girl received racial abuse in the Hunger games film......

This rule is clearly one way

It's context dependent but in theory there should be no basis for someone's intelligence or lack of it to be discriminated. If its genetics that cause intelligence, then when you insult a dumb person you are basically having a go a their DNA code and that's ridiculous and leaves you vulnerable to that person insulting even the slightest negative effect your DNA had had on you. If its environment than again it's not the individuals fault but is instead the fault of the adults responsible for ensuring that the environment for the person is conducive to learning. Infact, it may not even be their fault as the economic and social conditions present in a place ensure that access to supposed neccesities of intelligence increasing is limited if not impossible to get.

Why are people so quick to cite luck as the reason behind a lot of people's wealth and apparent intelligence but then fail to do the same for when someone is poor and not as intelligent? It works both ways and if the conditions in Oxford allow for young Donald to become rich, possibly through his Dad, then it's acceptable to assume that it's the conditions in Peckham which allow for young Marvin to become, or remain, poor and not anything to do with race or any other ridiculous notion and it's for this reason that people who criticise someone's intelligence have failed to realise this obvious truth; no one becomes dumb by choice

Apparently, discrimination on the basis of physical features like skin colour, height and attractiveness is frowned upon but it is okay to discriminate on the basis of intelligence. Why is that, given that physical features and intelligence are both genetically predetermined?

Because otherwise we'd live in a completely unproductive and ambitionless society.

You're not allowed to discriminate based on anything at all - to discriminate in this context simply means to take into account attributes - either real or probabilistic, that are not directly relevant to the decision at hand.

Specific and relevant differentiation on the other hand, has always been perfectly acceptable and always will.

Examples:

Specifically choosing a white person over a black person to play a white character in a film is fair differentiation because its a relevent attribute.
Specifically choosing a white person over a black person to be a security guard is discrimination because the attribute is not relevant.

Charging a higher car insurance premium to someone because they have a poor record of safety is fair differentiation because the attribute is specific to that person.
Charging a higher car insurance premium to someone purely because they are male is discrimination because the attribute is based purely on a statistical association.

What about the age discrimination when it comes to car insurance That's a statistical association.

Before someone comes back at me with a big well thought out argument, I'm just joking around.

Discrimination on age is against the law.
Differentiation based on driving experience is not.

The arguments are subtle and there is always a bit of a grey area, but the principles generally hold up to closer inspection.