Estimation of lethal equivalents in the Tajima beef cattle

Abstract

Although the Japanese Black cattle in Hyogo prefecture has been closed to the other subpopulations since early times, remarkable inbreeding depression has not been reported. However, more rapid increase of inbreeding will be unavoidable if genetic diversity of the population is conserved by the establishment of several strains, where mating is basically carried out within each strain. Therefore, information about the genetic load of the population should be valuable. In this study, lethal equivalents affecting mortality in early life were estimated for the population. Intercept (A) and slope (B) of regression developed by Morton et al. (1956) were estimated to be 0.058 and 0.111 by the weighted least squares technique, and 0.033 and 0.000 by maximum likelihood method. From these estimates, lethal equivalents of the population were estimated to be 0.222～0.338 and 0.000～0.066 per zygote, respectively. Relatively small number of lethal equivalents were considered to be responsible for the two reasons. The first one is that deleterious recessive alleles might be culled by the selection against recessive homozygotes frequently produced by inbreeding. The second one is that many deleterious alleles might be disappeared from the population by chance because of the strong selection for economic traits.