write specific code IF post is in a subcategory of X

How’s that for a title?

I’m trying to work out how I can write chunks of code on the post template (Single) that trigger when the post is within any sub-category of a specific category.

So, lets say my top level cat is “fruit” with sub-cat’s “apples” and “bananas”.

For any posts to the categories “fruit” or “bananas” (or any other categories under “fruit” only) I want to write some specific code elements.

Any pointers greatly appreciated.