How a trait determined by a small number of loci, each with two different alleles, can assume a continuous distribution of 1:2:1, 1:4:6:4:1 and 1:6:15:20:15:6:1 respectively. (Adapted from McGuffin, P., Owen, M.J., and Gottesman, I.I. (2002). Psychiatric Genetics and Genomics, Oxford University Press, Oxford.)