Code:
X Y
X XX XY
X XX XY
punnet square for gender shows that you have a 50/50 chance of male/female offspring. they get bigger when you have recessive traits and I haven't studied genetics in years, but saying you get your traits from your grandparents is as true as sayign you get them from your parents and as true as saying you get them from your great grandparents.
chromosomes (your genetic material) come in pairs (normally), and you get one from each parent.
[edit - I don't know what possesed me to type this next sentence, as it is totally off] as they got one of each pair from their parent, there is approximately 50% of your grandparents genetic code simply not present in your genes.
[/edit - that should be 75% shouldn't it?]
if we represent chromosome pairs as Aa = dad's dad, Bb = dad's mom, Cc = mom's dad, and Dd = mom's mom then your chromosomes will be
Code:
Aa Cc
B AB aB CB cB
b Ab ab Cb cb
D AD aD CD cD
d Ad ad Cd cd
so you've got 16 possible chromosome pairing outcomes, or a 1/16 predicted chance for any particular pair showing up in a grandchild. BUT once the children of the original generation are conceived, their genetic material is a matter of reality, not probability, so their offspring will NOT have the same chance as the punnet square for having a particular gene pairing.
or i could have just said punnet squares are for predictions of descendent's traits. they don't prove that you get your trait's from your grandparents, except in the sense that your parent's got their traits from your grandparents, and you got your traits from your parents, so yeah, you have a subset of your grandparent's gene pool.
i'm not sure that was very clear at all, and it's been a while so someone correct me if i made mistakes.