Mutations occur in sex cells or germ line cells are heritable and known as germline mutations. Sex cells produce gametes (sperm and ovum) and hence, mutation in these pass to the offspring. When either a mutated ovum or sperm come together to form a single-cell zygote, each cell in developing zygote has that mutation.
Thus, if a mutation affects the protein required for heart muscle in an adult rabbit, it will be pass to his offspring. The offspring will not be able to make functional proteins to build heart muscles and will die as heart is a vital organ.
