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A follow up question to When will we know if our child is left- or right-handed?.
Is handedness predetermined genetically? Or is it something the baby somehow learns, or maybe it is a result of some kind of activities it does?
And can ambidexterity be learned or is it a quality one has to be born with?
Yes, No, Both, and Debatable are all valid answers to the question. I suggest reading the Wikipedia article about handedness. It goes through several different theories about what the primary reason for handedness is, and links several pieces of research for those theories.
Whether handedness is genetic or not is debated and there isn't (as far as I know) any definite solid answer. Some research has shown that it might be related to a specific gene and other research has shown that the link isn't there.
Factors other than genetic could range from conditions pre-birth to early development after birth.
I know of many people who were left handed, and due to social stigma against left handedness (much more common in the past) 'trained' themselves to use their right hand for everything.
My brother is ambidextrous, though due to his early schooling he made a strong shift to using is right hand for most tasks.
Other people I know were right handed but taught themselves to use their left hand to write, throw, or bat (in baseball, a left handed pitcher and hitter can give a distinct advantage). That said, I would not consider these people to be ambidextrous, but rather to have become competent at certain specific tasks with their left hand.
That said, someone who strongly desired to use both their right and left hand to perform competently at most tasks could probably train themselves to do so. Most likely, they would perform better using their dominant hand, but can perform competently with their off-hand.
Handedness is definitely an inherited thing. My Dad taught me to write, to tie my shoelaces, and any of the sporty things I might do (like playing catch). He is left-handed, I am not. My handwriting was known for being atrocious when I was a kid and writing by hand was always very difficult for me until some right-handed teachers started to help. My shoelaces still wind up tied upside down.
There is a gene for left-handedness that has been discovered and links in with how people seem to use their brains/how their brains are wired (which relates to the next part of your question).
Brain imaging has shown certain connections between certain emotions and functions in the brain and where the brain is "on" or firing while completing certain tasks of feeling a particular emotion. While things are far more complex than outlined in this article by National Geographic on the matter, the article simplifies things to say that, for example, emotions are felt in the right side of the brain and speech and language are controlled by the left in right handers. A left-hander's brain is reversed from the right-handed so the structure of the brain is definitely different. How this all boils down to answering your question is, No left-handedness, or ambidexterity cannot be learned in the whole and purest sense of things. It isn't just about what hand you use for writing, but about the way the brain is wired.
Having said that: I injured my right hand badly when I was in college. The skin over it got shredded and a a few ligaments were severed. It was expected to heal, but I was told I would never be able to use it for fine-motor skills again and that I should learn to write, paint, etc. with my left hand. This was before laptops were affordable and when only the wealthiest had them so I was stuck taking notes in class etc. to the best of my ability with my left hand (I also got a mini tape-recorder, but listening to the tapes over and over again to find a note was cumbersome). I learned to write left-handed and I learned fast. My left-handed writing was certainly as good as my right-handed writing had been in third or fourth grade. I'm sure if I had continued writing with my left hand, it would have continued to improve further.
My hand has actually healed and I have much more complete use of it than expected and as soon as I could hold a pencil with my right hand - even though it was painful, I went right back to using it instead of the left because it was easier. I'd imagine any skill could be like that - if someone really wanted to be able to bat left-handed, with tons of practice, they could probably learn even if they are right-handed. They may not ever be quite as good left handed as right handed, but they could learn. The question is, is it worth the time and effort? In most cases, probably not.
