Inbreeding: One word, several meanings

The way geneticists use the word “inbreeding” can be somewhat puzzling. How can the same word be applied to situations as different as the presence of commong ancestors in the pedigrees of two individuals, on one hand, and the attitude towards marriage in a population, on the other? How can the same parameter, i.e., the “inbreeding coefficient F,” be used both, to measure the degree of genetic resemblance between individuals within a single population and also to measure the degree of genetic differences between groups of individuals.It will be useful for the sake of translation as well as for that of clarity, to delineate the various situations in which geneticists are led to refer to “inbreeding.”We shall see that this one word is used in, at least, five concepts: relationship between relatives; genetic drift; departure from panmixia of mating behaviour; subdivision of a population into several isolated groups; and divergence between the actual genotypic structure of a population and the reference “Hardy-Weinberg Structure.”