The usual views of hereditary transmission of qualities from parent to child have been contentedly vague as to the exact amount derived from either parent or grandparent or more remote ancestor. It has been probably assumed by most of those who have given any thought to the subject that such calculations were impossible or that at least that the laws governing such transmission depended upon so many and so different conditions as yet, and probably long to be, unknown to us, that it was useless to try to seek them out. We know as physicians that certain pathologic heredities are more or less special to one or the other sex, that in some prominent instances a feature or cast of countenance, mental or moral trait seem to be a family characteristic, while in others there is apparently nothing to indicate descent in the way of family traits or features. Once in a while a known ancestral character makes its appearance, having skipped one or more generations, and to this we give the name of atavism and leave further explanation as useless or impracticable. The hitherto recognized laws of heredity are few and hardly touch the question on its quantitative side; the distribution of ancestral traits has been considered to be too capricious as a rule to afford the data for generalizations in this particular direction, at least in the human species, where all matters involving faculty or capacity are so much more complex than in the lower animals.
Quantitative Heredity. JAMA. 2014;311(8):866. doi:10.1001/jama.2013.279355