Genetic and environmental factors in the longitudinal growth of rats: III. Craniofacial shape change.

This investigation expresses the similarity of the craniofacial shape of rats in terms of a single parameter and determines to what extent the shape variation of the craniofacial complex is genetically determined. To quantify the similarity of the craniofacial shape between any two individuals, diagrams plotted from their lateral cephalograms are so oriented that the distance from the points on the one to the corresponding points on the other is minimized. The data consist of measurements from within-strain, between-strain, maternal half-sibs and paternal half-sibs groups. The average dissimilarity is computed in each group and compared. The results indicate that this method can be used to estimate the similarity of the craniofacial shape. The dissimilarity of within-strain pairs shows the smallest Dh value, whereas between-strain pairs have the largest Dh value. Those of maternal and paternal half-sib pairs show the intermediate Dh value. The value of dissimilarity for these four pairs tends to decrease gradually with age and a considerable genetic effect acts on the change of the craniofacial shape during growth, while the maternal effect does not act significantly on the change of the craniofacial shape during growth.