The information passed from parents to offspring is coded in DNA molecules. Some new gene combinations make little difference, some can produce organisms with new and perhaps enhanced capabilities, and some can be deleterious.
Genes are segments of DNA molecules.
Inserting, deleting, or substituting DNA segments can alter genes. An altered gene may be passed on to every cell that develops from it. The resulting features may help, harm, or have little or no effect on the offspring s success in its environment. Gene mutations can be caused by such things as radiation and chemicals. When they occur in other cells, they can be passed on to descendant cells only.