An inherited change in the genes as inferred from an inherited change in the appearance of the offspring. The term can be extended to include genetic changes occurring in somatic cells (somatic mutations). The latter can result in abnormalities of growth such as *chimaeras
. They are not inherited unless the mutant tissue gives rise to a reproductive shoot. There are two main categories of mutations: *gene mutations
, which are alterations in a single gene; and *chromosome mutations
, which may involve large numbers of genes. Mutations are characteristically rare, harmful, and recessive. The frequency of spontaneous gene mutations differs with different gene loci. For example in maize, the mutation giving rise to shrunken endosperm occurs about once in every million replications of the gene locus. However the mutation giving a colourless aleurone layer occurs more frequently, about 110 times in every million replications of the locus. Reversions of a mutant gene to a normal gene occur less frequently. The low frequency of spontaneous gene mutations may be attributed to the great stability of DNA. One reason for the occurrence of spontaneous mutation is that all the bases can also exist in very rare isomeric forms. Thus while adenine normally pairs with thymine, in its rare isomeric form adenine will pair preferentially with cytosine. Over two replications this could result in a spontaneous switch from an A:T pair to a G:C pair in the DNA. The low spontaneous frequency of mutation can be increased by various agents called *mutagens
. The harmful effect of most mutations is due to the fact that they occur randomly. A purely random change to a normal allele (gene) is more likely to result in a defective protein than one that confers a selective advantage on its owner. For similar reasons most mutations are recessive. In a heterozygote the effect of a mutant allele that produces a nonfunctional protein is likely to be masked by the normal allele, which produces a functional protein. The former would therefore be described as recessive and the latter dominant. Despite their infrequent occurrence and their frequently deleterious effects, mutations are of considerable biological significance because they are the ultimate source of all genetic variation. The occurrence of different alleles, e.g. Τ (tall peas) and t (short peas), of the same gene (pea height) may thus be attributed to mutations occurring sometime in the history of the species.