It is possible for brown-haired parents to have a blond baby if both parents carry the gene for blond hair and both pass it on to the child. This happens in about one out of every four cases where the parents both carry the blond gene.
Blond hair is a recessive physical trait, which means that it does not appear in an offspring if there is also a gene for dark hair present. This means that individuals can carry the gene for blond hair without physically manifesting it. Children receive one gene responsible for hair color from each of their parents, and if both of these are recessive blond hair genes, the child is blond.