human biology - How are the gene sequences of individual sperm and egg cells "randomized"?

I think we can attribute this to sheer probability. The human genome contains around 3 billion base pairs. When you consider recombination from chromosomal crossover that occurs in germ line cells, there is an astronomically huge number of possible unique combinations that can be made.

Of course, males generate many many sperm, so some of these are bound to be similar. But the number of sperm produced by any individual doesn't come anywhere close to the number of possible unique sperm that he could produce.