Phenotypic variability of inbred and outbred mice

Abstract

THE relative phenotypic variability of inbred and commercially available outbred stock mice has never been satisfactorily investigated, even though it has important implications in the design of animal experiments. Early workers¹ suggested that “the published facts suggest that inbred strains are usually more variable than inter-strain hybrids, with random bred colonies occupying an intermediate position.” Later it was concluded that with respect to phenotypic variability, “no general law can be asserted on the comparison of inbred strains with randombred stocks”². A subsequent debate on the suitability of inbred, outbred and F₁ hybrid mice for biological assay^3–6 failed to clarify the situation, and many research workers were left with the impression that inbred animals are more variable than outbred ones. Much of the early work could be criticised on the grounds that sample sizes were small, and few populations were sampled. We have collected extensive data on the phenotypic variability of the shape of the mandible, as part of a routine genetic quality control programme^7,8, and have found that assuming equal sensitivity to an experimental treatment 50% more outbred than inbred mice would be required to detect a 1-unit change in mandible shape.