Control of Embryonic Growth Rate and Fetal Size in Mammals

All around us in our everyday life is evidence that the size and growth rates of mammals are under genetic control. We are all familiar with very small and very large dogs and do not expect to see either an adult Great Dane peering from a befurred lady’s bag or a Chihuahua dragging its owner for a walk. Such comments as “He’s big for his age” bear witness to our knowledge that growth toward adult stature follows a predictable time course. Adult size in mammals is thus said to be determinate and in general is achieved some time, perhaps many years, before death. This chapter is concerned for the most part with the genetic determinants involved in growth in a nonpathological setting. The focus is on the earliest stages of development at which differences in growth rate can be assessed and investigates what sorts of controls or constraints may be operative at that time. First of all, then, when do genetic differences in growth rate appear?