A mutation that can be inherited by offspring would result in

A base substitution in gametes during meiosis

The only way a mutation can be inherited is if it is carried by a gamete produced by a mutated germ cell in the gonad. Only the germ cells undergo meiosis to produce the haploid cells that pass down to the next generation.

A change in any somatic cell will not be passed to the next generation as these cells only reproduce by mitosis. All mutagenic agents that impact somatic cells produce changes in the daughter cells in that tissue only. Thus most mutagenic agents are carcinogenic in somatic cells.

(taken from a user on Yahoo)