Genetic transformation for egg and feather colour hs been obtained in the whole animal, chicken, by the use of irradiated male gametes. In this technique recipient females are first inseminated with irradiated semen from the donor and 24 h later are reinseminated with unirradiated, normal semen of the recipient strain. The transformed progeny are identical to the female strain but for one or a few genes derived from the irradiated semen. The expression of the transferred gene is irregular and may occur either in the first generation, after insemination with irradiated semen, or may occur in the following backcross generation. It is suggested that integration of a normally recessive, transferred gene at non-homologous sites in the recipient chromosomes might affect not only its regularity of expression but also its dominance relationship with the original maternal gene, "releasing" it from the dominance of the latter. The technique potentially offers a rapid method of improving an already established strain of an animal.