Hi Guido. Everyone gets “about half” of their genes from each parent, but it’s not quite exact. We have 23 pairs of chromosomes in each cell, which are the big molecules that contain our genes. One set of 23 we get from mum, the other from dad. 22 of these match up exactly, but the last pair don’t always: we call these sex chromosomes because they decide whether a baby turns out to be a boy or a girl. They come in two varieties called “X” and “Y”, of which X is slightly smaller. All of mum’s egg cells have an X, and so do half of dad’s sperm, the rest having “Y”. So depending on which type of sperm fertilises the egg, the resulting embryo will have either XX and become a girl, or XY and become a boy. So overall, a girl will have exactly half her genes from each parent, but a boy will have slightly more from mum, as the Y he gets from dad is smaller and has fewer genes than the X he gets from mum.
Even though there are fewer genes on the Y chromosome, it’s still very important, because the genes it does carry sets off the sequence of events inside the embryo that tells it to become a boy.