The basic idea is simple. Imagine each parent has pairs of identical genes,
call them AA and BB.

AA + BB = AB + AB + AB + AB (four combinations, but they're all the same).
This is the F1 generation.

Now cross the AB F1s...

AB + AB = AA + AB + BA + BB or AA + 2 x AB + BB (AB and BA are the same).

Note that half the offspring match the parents, the other half match the
grandparents (AA and BB).

Now throw in some random polination from your neighbours plants, nature's
natural "randomize the odd gene and see what happens" etc. and you see what
the F2 generation is a right mix.

Paul DS.