The n/2 model (pronounced either ‘en over two’ or 'half en') suggests the mode of operation of the gene-for-gene relationship in a wild plant pathosystem. It apparently functions as a system of locking in which every host and parasite individual has half of the genes in the gene-for-gene relationship (i.e., n/2 genes, where n is the total number of pairs of genes in that relationship). Each gene in the host is the equivalent of a tumbler in a mechanical lock, and each gene in the parasite is the equivalent of a notch on a mechanical key. Provided that each n/2 combination of genes occurs with an equal frequency, and with a random distribution, in both the host and parasite populations, the frequency of matching allo-infections will be reduced to the minimum. For example, with six pairs of genes, each host and parasite individual would have three genes, and there would be twenty different locks and keys; with a twelve-gene system, there would be 924 six-gene locks and keys. Given an equal frequency and a random distribution of every lock and key, the frequency of matching allo-infection would be 1/20 and 1/924, respectively. These figures are obtained from the binomial expansion illustrated by Pascal's triangle.