Lots of theories about generation up to the XXVII century base themselves on the marvelous power of parents' imagination - the mother's above all - of conveying resemblance to the foetus or conditioning its shape till to the extreme monstruosity. For a long time mother's fancies explain the appearance of naevus, cravings, skin spots, hair colour, congenital deformities and so on. We can recognize one of the grounds of that idea, perhaps the most authoritative, in a biblical passage having Jacob the Patriarch as protagonist. This essay deals with history of the imaginationist theory from Jacob's episode and its following interpretation, with a particular attention for its influence on eugenic doctrines in fashion during the centuries XVI and XVII.