Can a mutation be passed down, or does it start and end with the organism?

Somatic mutations occur in non-reproductive cells and won't be passed onto offspring. Its seeds will not carry the mutation. The only mutations that matter to large-scale evolution are those that can be passed on to offspring. These occur in reproductive cells like eggs and sperm and are called germ line mutations.

Explanation:

Some mutations are hereditary because they are passed down to an offspring from a parent carrying a mutation through the germ line, meaning through an egg or sperm cell carrying the mutation. Mutations can introduce new alleles into a population of organisms and increase the population's genetic variation.