A study of some aspects of growth and reproduction in two inbred lines of mice and their crosses : a thesis presented in partial fulfilment of the requirements for the degree of Master of Agricultural Science in Massey University of Manawatu

Abstract

The mating of closely related individuals is generally believed to result in inferior offspring. Recognition of this danger has been suggested as the reason for evolution of human social customs. However, historical researches suggest that inbreeding unavoidably occurred in small, isolated, ancient communities, and marriage of close relatives was encouraged in the Egyptian, Greek and Hebrew civilisations (Zirkle, 1952). Inbreeding is likely to have occurred in the isolated flocks of nomads, but it is probable that inbreeding depression, as such, was not recognised until the eighteenth century when the early livestock improvers are reputed to have found that the fertility of their animals deteriorated with continued inbreeding. Hybrid vigour from cross breeding plants was described by Koelreuter (1766) and confirmed by later botanists., .After much detailed work, Darwin (1876) concluded: " •••cross fertilisation is generally beneficial and self fertilisation injurious." and that (a) Mechanisms exist widely for the avoidance of inbreeding. (b) Inbreeding has effects likely to incur selective disadvantages. No suitable hypothesis could account for the usually deleterious effects of inbreeding, or the reverse phenomenon of hybrid vigour until the rediscovery of Mendel's work in 1900. The effects of inbreeding can now be explained in terms of the dominance and recessiveness of genes. (Falconer, 1960). The aim of experiments reported in this thesis was to study growth, reproduction and mortality in two inbred lines of mice, derived from common ancestors, to find if differences between them had arisen during inbreeding. Reciprocal crosses and matings of the F1 progeny of these crosses were made to find if hybridisation of the parent lines led to improvements of these charact­eristics. This study was suggested by Drs. D.S.. Flux, M.F. McDonald and R.E. Munford to whom the author is indebted for advice and guidance. The author also wishes to acknowledge the assistance of Mr. E. Ormsby in histological preparations, of Mr. P .S. Dale in photomicrography, and of Mr. C. Muir who drew some of the figures reproduced in this thesis. Thanks are also due to the staff of the University Library for their efforts in obtaining literature. [From Preface