[lmm]

In general you can say that on average a child has 25% of their genes from each grandparent. But in the case of the sex chromosomes, we know that a male has 50% of their paternal grandfather's sex chromosomes and 0% of their paternal grandmother's sex chromosomes (and vice versa for a female); the Y chromosome is passed on whole or not at all (you can't get half of it from your mother).