How can I describe someone who feels little or no emotion?

I don't mean someone who lacks emotion because they "don't care", but because either they can't feel emotion or the emotional response is delayed because of a genetic disposition.

Maybe there is an appropriate medical term that could be used.

The word stoic is recommended from a similar question, for example - but that question and its answers relate to an individuals ability to endure/tolerate a situation or simply ignore their emotions.

Blunted affect may be the noun, but if you're looking for an adjective to describe someone like that the term is affectless.

affectless : showing or expressing no emotion; also : unfeeling

The medical term is blunted affect. A more extreme case is called a flat affect.

From Wikipedia:

Blunted affect is the scientific term describing a lack of emotional reactivity on the part of an individual. It is manifest as a failure to express feelings either verbally or non-verbally, even when talking about issues that would normally be expected to engage the emotions. Expressive gestures are rare and there is little animation in facial expression or in vocal inflection

Reminds me of a line in Shawshank Redemption.

You strike me as a particularly icy and remorseless man, Mr. Dufresne.

"Cold" (or more poetically, "Icy", as used in the movie) can mean you are not easily affected emotionally, and do not show emotions.