Contrary to the verse by the great population geneticist J. B. S. Haldane, written just after being treated for the cancer that was to take his life, his was among the first generations in which a lot of chaps were bumped off by cancer. Previously most people died from other causes, such as infectious diseases. Although cancers are genetic diseases, selection has prevented most cancers from being directly heritable, but cancer demonstrates the important effects that somatic mutations can have. Cancer reflects evolution in a microcosm, occurring among our own cells during our own lifetime rather than among the individuals in a population over evolutionary time.

Cancer age patterns reflect the time required for somatic mutations to produce their complex cellular phenotypes. This leads naturally to a general consideration of the age patterns of chronic disease and, consequently, of aging in general. These are the subjects of this chapter.

Somatic mutation: the genotype changes with age

An individual begins life as a single cell, but an adult organism is the product of a very large number of subsequent cell divisions that occur as the zygote grows, develops, and renews its tissues. Somatic mutations occur in these somatic cells, and are inherited by their mitotic descendents during the life of the individual.