Some of our basic ideas about genes and their transmission have been established for nearly a century. One of these is Mendel's Law of Segregation, from which we derived the transmission probabilities (τs) given in Chapter 5. However, as noted earlier in this book, some investigators estimate these probabilities from the general likelihood in families rather than specifying them, and the estimates are sometimes significantly different from the mendelian values of 0, ½ and 1. A second basic principle of classical genetics is that genes are faithfully transmitted during mitosis, so that all of our somatic cells have the original inherited genotype. The process of somatic rearrangement in immunoglobulin and TcR genes shows that this postulate, also, is not always correct.

This chapter shows various other ways in which violations of these classical assumptions have important effects on the pattern of genotypes in families and the relationships between genotype and phenotype. The term non-traditional inheritance is often used for such phenomena {Hall, 1990a,b; Holliday, 1987, 1989; Solter, 1988}, but the mechanisms have always been there whether part of our ‘traditions’ or not!