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In addition to @mgkrebbs response, I'd add that many genes are controlled by epigenetic mechanisms which may mask one of the parental copies by methylation. This can be a specific effect where, the active gene is specifically from either the father or the mother. Mosaicism, where one copy of gene is randomly inactivated in any given cell, is also possible. ...

First, while half the chromosomes come from each of the two parents, these two sets of chromosomes are not termed X and Y (they would usually be called maternal and paternal). The terms X and Y refer to potential members of just one pair of the 23 pairs (in humans) of chromosomes, and X chromosomes can come from either the mother or the father. The ...