About 1/10 of the worlds population are left handed. Archeological evidence shows that its been the way for as long as 500,000 years. With about 10% of human remains showing the associated differences in arm length and bone density and some ancient tools and artifacts showing evidence of left hand use. It can be predicted even before birth based on the fetus position in the womb.
Identical twins, who have the same genes can have different dominant hands. In fact, this happens as often as it does with any other sibling pair. But the chances of being left handed or right handed are determined by the handedness of your parents.
If your father was left handed but your mother was right handed you have a 17%chance of being born left handed.
The benefits of being left handed are clearest in activities involving and opponent like competitive sports. For example, 50% of top haters in baseball have been left handed.