VARIANCES OF ADDITIVE AND DOMINANCE GENETIC-EFFECTS FOR OVULATION ANDTWINNING RATES IN A POPULATION SELECTED FOR TWINNING

Estimates of variances due to additive and dominance genetic effects a nd permanent and temporary environmental effects were obtained for ovu lation and twinning rates from a composite population selected for twi nning rate. Measures of ovulation rate after 11 mo of age on 2,317 hei fers with a total of 19,209 measures were used. Twinning measures were on 1,522 first-parity cows, 1,311 later-parity cows with a total of 3 ,571 measures, and 1,704 all-parity cows with 5,100 measures. Models i ncluded fixed effects of year-season-age at calving for twinning, and year-season of birth, age in months, and calendar month of measurement for ovulation rate. Four analyses were performed for each sample: com binations of models with and without dominance effects and with and wi thout covariates for fractions of inheritance from the seven foundatio n groups. Variance components as fractions of phenotypic variance for analysis of all ovulation rate measures were .076, .000, and .045 for additive, dominance, and permanent environmental effects with no found ation groups in the model and .069, .000, and .050 with foundation gro ups in the model. For sums of eight measures, the estimates were .287 and .000 for relative variances of additive and dominance effects with groups in the model and .316 and .000 with groups ignored. For twinni ng rate for first parity, estimates were .126 and .209 for relative va riances of additive and dominance effects; for later parities, estimat es were .045 and .035 for models including foundation group effects. T he results suggest lack of dominance effects in expression of ovulatio n rate and the possibility of dominance effects for embryo and(or) fet al survival or conception rate because twinning rate is a function of ovulation, conception, and embryo and(or) fetal survival rates.