Genetic and phenotypic consequences of inbreeding.

The effects of inbreeding on heterozygosity, mean performance, variation within and between lines, and fixation or loss of genes are discussed. Theoretical predictions for degree of pedigree inbreeding and consequences for genes neutral with respect to fitness are outlined first, and then the likely modifications necessary to account for natural selection during inbred line development are considered. For illustration, some results from experiments with laboratory animals are reviewed. Inbred lines or F1 crosses between them are special genotypes, not representative of the population from which they were derived. Similarly, outbred stocks of laboratory animals that have a long history in the laboratory, or narrow base, or are maintained in small numbers, will lack variation and become like inbred lines. The efficacy of using either inbreds or such outbred stocks in toxicological screening is questioned on the ground that the range of genotypes is inadequate.