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A Phenotypic Expression of Homozygous Dwarfism in Beef Cattle^{1, 2,}

University of California^,3

Abstract

A study of head contours of over 60 dwarfs and several hundred normal animals of the Hereford, Angus, and Shorthorn breeds reveals that the outstanding characteristic of the specific type of dwarfism studied is a brachycephalic head with a marked mid-forehead prominence present at birth and persisting throughout life. This type of contour is discontinuous with and does not intergrade into the type of contour of normal homozygotes. Accompanying this anomaly is a misshapen mandible in which there is marked malocclusion of the incisors with the dental pad. Incisors may extend forward to the dental pad from one-half to three and one-half centimeters. These characteristics are also symptoms of thyroid deficiency. Moreover, the stunted growth, heavy breathing, and extreme pot-belly of the dwarfs are manifestations of thyroid deficiency characteristic of hereditary or experimentally induced cretins. These morphological features are in agreement with physiological studies completed in our laboratory being reported in another paper which prove conclusively that dwarfs of this type possess a gross deficiency of the thyrotrophic hormone.

¹ This work was supported by the Kellogg Fund for the application of genetics to farm mammals.

² A part of the cost of the illustrations was borne by a grant by Mr. C. M. Goethe.

³ Since the data were collected in the field, the authors are deeply indebted to a great many people. Although they prefer to remain anonymous, breeders of all the three breeds involved gave their time and helped in every way possible to make this study a success. The interest, aid, and cooperation given by various personnel of some of the breed associations were most helpful. We are obligated to Professor E. B. Stanley and Mr. O. F. Pahnish of the University of Arizona for permitting us to measure the dwarfs they were investigating. Entree to most of the dwarfs studied from California herds was obtained through the University of California Agricultural Extension Service. This was made possible by the genuine interest of Director of Agricultural Extension, J. Earl Coke, Director of Programs J. J. McElroy, and Extension Animal Husbandmen Reuben Albaugh and H. T. Strong. The Farm Advisors in all the counties in which studies were made were most cooperative. We are especially obligated to Mr. R. C. Geiberger, Farm Advisor of Sacramento County. In the laboratory we were most fortunate in having the assistance of Mrs. Ruth Skolil.

⁴ Davis, California. This study was in cooperation with the Bureau of Animal Industry, U.S.D.A. and state experiment stations under Western Regional Project W-l on beef cattle breeding research.