Genetic variation and craniofacial growth in inbred rats

There has been no experimental study clarifying the contribution of genetic variation to craniofacial growth on a longitudinal basis, although its importance is generally accepted. Utilizing diallel crosses of five strains of inbred rats, the present study concerns an estimation of the contribution genetic variation makes to the longitudinal change of craniofacial size. The overall sizes of craniofacial complex components of F₁-offspring rats were investigated postnatally by means of roentgenographic cephalometry, and quantitative genetic analysis was performed by the method of Wearden [1964]. It was revealed that the relative contributions of the genetic and environmental components to total variance in craniofacial size varied with age and that the genetic component of variance significantly increased until the 80th day. On the other hand, the maternal component of variance showed the maximum value during the early periods of postnatal growth (ie, the 10th to 25th day), gradually declining thereafter to a very small amount by the 80th day. The environmental component of variance increased slightly and gradually with age through the experimental period. In conclusion it appeared that genetic variation became more significant during longitudinal growth of the craniofacial complex as the maternal effects diminished.