Figure 1. Gene mutations. (a) A GCAT mutation caused by guanine shifting from its common (keto) form to a rare enol form at the time of DNA replication. This enol form pairs with thymine instead of cytosine. During the subsequent replication, the newly incorporated thymine pairs normally with adenine. It is also possible for guanine to convert to its rare enol form during its incorporation opposite a template strand, rather than at the time of replication (not shown). In this case, guanine will be located opposite a thymine and replication will result in an ATGC mutation. (b) Addition and deletion mutations caused by strand slippage. On the left, addition mutations are caused by slippage of the newly synthesized strand. On the right, deletion mutations are caused by slippage of the template strand. Repetitive bases (A on the left, CT on the right) stabilize the looped out portion of the DNA. (Adapted from Gardner and Snustad (1984) Principles of Genetics, New York: Wiley; Griffiths et al. (1996). An Introduction to Genetic Analysis, New York: WH Freeman.)

Figure 2. Chromosome mutations. (a) Structural mutations. Wavy arrows in the diagrams for terminal and interstitual deletions indicate an agent such as ionizing radiation. (b) Numerical mutations during meiosis. Aneuploid gametes are firmed by nondisjunction; that is, homologues (far left) do not separate and move to opposite poles. Nondisjunction can occur during the first (upper diagrams) or second (lower diagrams) meiotic division. Circles within chromosomes indicate centromeres. Nondisjunction may also occur during mitosis, in which case the sister chromatids do not properly move to opposite poles. (Partly adapted from Griffiths et al. (1996). An Introduction to Genetic Analysis, New York: WH Freeman.)