Makes perfect sense
Your mother has two "ginger" hair-colour genes.
Your father has one "dark" and one "blond" hair-colour gene.

Your mother passed on a ginger gene to you (of course - there was no other choice)
Your father passed on his blond gene to you. There was a 50/50 chance.
You therefore have one blond, and one ginger hair-colour gene.

Your mother is ginger because two ginger genes = ginger hair.
Your father is dark-haired because a dark gene overrides a blond gene.
You are blond, because a blond gene overrides a ginger gene.

That means that - although you are blond yourself - you are a "carrier" of the ginger gene. If you had a child with a ginger lady, there'd be a 50% chance of it being ginger. If you had a child with a blonde or dark-haired lady, then there would be either a 25% chance of it being ginger, or a zero percent chance - depending on whether she was also a ginger carrier.
(EuroSong I love horse meat: best of all the animals, Tue 2 Mar 2010, 19:03, Reply)