The effect of inbreeding upon growth in Jersey cattle.

AST and JONES (1919) and CASTLE (1930) reviewed the significant experiE ments with plants and laboratory animals that contributed to the formulation of the modern viewpoints concerning the effects of inbreeding. The phenomena associated with inbreeding: decline in vigor, increased uniformity within the inbred stock, increased prepotency in outside crosses, isolation of abnormal types, etc., were shown to be consequences of the increase in the proportion of homozygous gene pairs. While the inbreeding results obtained with laboratory animals are believed by geneticists to apply to cattle in a general way, such beliefs, representing, as they do, an extrapolation to relatively unknown genetic material, must of necessity be tentative until a sufficient number of adequately controlled experiments dealing with the effects of inbreeding upon cattle have been reported. The literature dealing with inbreeding experiments with cattle covers only the last two decades and is not extensive. This situation is understandable since perfectly regular inbreeding systems are not feasible for cattle. Furthermore, economic considerations and the relatively great time intervals between generations (four to five years) hamper the design of adequately controlled experiments. These obstacles have been materially reduced within the last three decades by means of improved statistical techniques and by WRIGHT'S (1922) publication of a method of calculating the coefficient of inbreeding which is strictly proportional to the probable loss in heterozygosis as a result of inbreeding alone. This inbreeding coefficient can be calculated for pedigrees exhibiting any degree of irregularity. In general, the reported experiments dealing with the effect of inbreeding upon various size characteristics in dairy cattle have suggested that inbreeding exerts a depressing affect upon these characteristics. WOODWARD and GRAVES (1933) found that intense inbreeding of grade Guernsey and grade Holstein-Friesian cattle caused a decrease in birthweight, a retardation in the growth rate of calves, and a reduction in mature weight. However, the reduction in mature weight was not proportionally as great as