Quantitative Genetics of Skeletal Variations in the Mouse. I. Crosses Between Three Short-Ear Strains (P, NB, SEC/2)12

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Abstract
Three short-ear strains of mice differ with respect to the frequency distributions of numbers of presacral vertebrae. In 384 mice of the P. strain, 92.2% had 25 presacral vertebrae, 3.4% had 26, and 4.4% had asymmetrical 25-26 intermediate vertebrae. In 245 mice of the NB strain, 99.2% had 26 presacral vertebrae, and 0.8% had 26-27 intermediate vertebrae. In 941 mice of the SEC/2 and 2d strain, 14.7% had 26 presacral vertebrae, 67.9% had 27, and 17.3% had 26-27 intermediate vertebrae. Crosses between the strains, P x NB, NB x SEC/2, P x SEC/2, expanded where possible to 1st, 2d, and 3d hybrid generations, and to 1st, 2d, and 3d backcross generations to both parental strains, produced frequency distributions of numbers of presacral vertebrae which approximately conform to distributions of a multiple factor gene model with thresholds.