These are genetically homozygous and fully inbred strains. Variation within an inbred strain is due to environmental effects and developmental noise. No systematic effort was taken to minimize environmental differnces.
Animals are of mixed sexes and ages.
The F1 hybrids are heterozygous at many gene loci, but members of a given cross are also genetically identical. Consequently, variation among members of a particular cross is environmental in origin. The F1's were generated by a cross between a low and a high cell number strain. The MidPar column is the midpoint between the low and the high strain. Maternal effect can be assessed by comparing reciprocal F1 crosses.