Size inheritance in the mouse

Abstract

In the wild, species of animals have a characteristic body size with limits which are rarely transgressed. As D'Arcy Thompson (1942) notes: "We are accustomed to think of magnitude as a purely relative matter. We -call a thing big or little with reference to what it is wont to be, as when we speak of a small elephant or a large rat." Yet, absolute size is very important; all D'Arcy Thompson's work shows that it is not enough to measure the relative dimensions of organisms for it is differences in absolute, and not relative, size which can explain much of the diversity in form observed between different species. The study of ailometry, as developed by Huxley (1932), has shown that differences in morphology used - for the taxonomic purpose of separating species may result merely from changes of body size.The genetic variability found within species is generally accepted as the variability which through the agency of natural selection -ult_i laately produces divergent species. In the course of evolution many groups of animals and especiallyvertebrates show a more or less steady increase of body size (Cope's Rule cited by Rensch, 194e). An understanding of the manner in which size is inherited is therefore fundamental to a study of speciation. In domesticated species the replacement of na iral selection by artificial selection has resulted in much larger intra- specific differences in size. An understanding of size can therefore explain much livestock improvement in the past and at the same time provide techniques for making possible much greater improvements in the future. The investigation of size inheritance is thus important both for fundamental theory and for its practical application.Individuals in a population cannot usually be separated into distinct classes with different sizes: instead, variation is observed to be continuous about a mean value. Each individual is characterised by a measurement which, if the gradations of the measuring instrument are sufficiently fine, is potentially unique. Size has thus to be treated as a quantitative character subject to the laws of quantitative inheritance. It is therefore necessary to give a brief review of current theories of quantitative inheritance before proceeding to a consideration of our knowledge on size inheritance.