The original DNA sequence when ,transcribed and translated would yield five successive valine residues. But the altered sequence would correspondingly read one aspartate and four serine residues. Mutations resulting from the addition or deletion of base pairs are called frameshift mutations. So far we have discussed the several ways in which the genetic material could be altered. Mutations could be either spontaneous or induced. In recent years there has been a more clear understanding of the causes of mutational process. Tautomeric shifts, ultraviolet radiations, chemicals such as hydroxylarnine, nitrous acid and a variety of alkylating agents can cause permanent alterations of nitrogenous bases. Aromatic compounds such as acridines can cause addition or deletion of one to more than twenty bases in DNA. We have briefly discussed mutations, a way or source of variability in natural populations. Together with recombinations occurring'at the tjnie of meiosis they form the raw materials for selection to act on. Before we discuss briefly about recombination, let us have a quick look at mutation rates in organisms.