mice were randomly assigned mates so long as the selected mate was not a sibling. This procedure was chosen to produce the fastest rate of response to selection at the expense of somewhat higher rates of inbreeding. After being selected from the initial, common F3 parental population, each line and replicate was a closed breeding population in which all offspring were tested at each generation of selection, with the highest scoring HEB or lowest scoring LEB mice selected as breeders for each subsequent generation of selection. Six breeder pairs were maintained in each line and replicate throughout the experiment.
